Add the concept of slaves capabilities to Redis, the slave now presents
to the Redis master with a set of capabilities in the form:

    REPLCONF capa SOMECAPA capa OTHERCAPA ...

This has the effect of setting slave->slave_capa with the corresponding
SLAVE_CAPA macros that the master can test later to understand if it
the slave will understand certain formats and protocols of the
replication process. This makes it much simpler to introduce new
replication capabilities in the future in a way that don't break old
slaves or masters.

This patch was designed and implemented together with Oran Agra
(@oranagra).

In previous commits we moved the FULLRESYNC to the moment we start the
BGSAVE, so that the offset we provide is the right one. However this
also means that we need to re-emit the SELECT statement every time a new
slave starts to accumulate the changes.

To obtian this effect in a more clean way, the function that sends the
FULLRESYNC reply was overloaded with a more important role of also doing
this and chanigng the slave state. So it was renamed to
replicationSetupSlaveForFullResync() to better reflect what it does now.

This commit attempts to fix a bug involving PSYNC and diskless
replication (currently experimental) found by Yuval Inbar from Redis Labs
and that was later found to have even more far reaching effects (the bug also
exists when diskstore is off).

The gist of the bug is that, a Redis master replies with +FULLRESYNC to
a PSYNC attempt that fails and requires a full resynchronization.
However, the baseline offset sent along with FULLRESYNC was always the
current master replication offset. This is not ok, because there are
many reasosn that may delay the RDB file creation. And... guess what,
the master offset we communicate must be the one of the time the RDB
was created. So for example:

1) When the BGSAVE for replication is delayed since there is one
   already but is not good for replication.
2) When the BGSAVE is not needed as we attach one currently ongoing.
3) When because of diskless replication the BGSAVE is delayed.

In all the above cases the PSYNC reply is wrong and the slave may
reconnect later claiming to need a wrong offset: this may cause
data curruption later.

Backported from Disque.

In Redis MULTIWORDCOMMANDNAME are mapped to functions where the command
name is all lowercase: multiwordcommandnameCommand().

1. We no longer use a fake client but just rewriting.
2. We group all the inserts into a single ZADD dispatch (big speed win).
3. As a side effect of the correct implementation, replication works.
4. The return value of the command is now correct.

This commit simplifies the implementation in a few ways:

1. zsetScore implementation improved a bit and moved into t_zset.c where
   is now also used to implement the ZSCORE command.

2. Range extraction from the sorted set remains a separated
   implementation from the one in t_zset.c, but was hyper-specialized in
   order to avoid accumulating results into a list and remove the ones
   outside the radius.

3. A new type is introduced: geoArray, which can accumulate geoPoint
   structures in a vector with power of two expansion policy. This is
   useful since we have to call qsort() against it before returning the
   result to the user.

4. As a result of 1, 2, 3, the two files zset.c and zset.h are now
   removed, including the function to merge two lists (now handled with
   functions that can add elements to existing geoArray arrays) and
   the machinery used in order to pass zset results.

5. geoPoint structure simplified because of the general code structure
   simplification, so we no longer need to take references to objects.

6. Not counting the JSON removal the refactoring removes 200 lines of
   code for the same functionalities, with a simpler to read
   implementation.

7. GEORADIUS is now 2.5 times faster testing with 10k elements and a
   radius resulting in 124 elements returned. However this is mostly a
   side effect of the refactoring and simplification. More speed gains
   can be achieved by trying to optimize the code.

Now used both in geo.c and t_zset to provide ZSCORE.

Current todo:
  - replace functions in zset.{c,h} with a new unified Redis
    zset access API.

Once we get the zset interface fixed, we can squash
relevant commits in this branch and have one nice commit
to merge into unstable.

This commit adds:
  - Geo commands
  - Tests; runnable with: ./runtest --single unit/geo
  - Geo helpers in deps/geohash-int/
  - src/geo.{c,h} and src/geojson.{c,h} implementing geo commands
  - Updated build configurations to get everything working
  - TEMPORARY: src/zset.{c,h} implementing zset score and zset
    range reading without writing to client output buffers.
  - Modified linkage of one t_zset.c function for use in zset.c

Conflicts:
	src/Makefile
	src/redis.c

Bug as old as Redis and blocking operations. It's hard to trigger since
only happens on instance role switch, but the results are quite bad
since an inconsistency between master and slave is created.

How to trigger the bug is a good description of the bug itself.

1. Client does "BLPOP mylist 0" in master.
2. Master is turned into slave, that replicates from New-Master.
3. Client does "LPUSH mylist foo" in New-Master.
4. New-Master propagates write to slave.
5. Slave receives the LPUSH, the blocked client get served.

Now Master "mylist" key has "foo", Slave "mylist" key is empty.

Highlights:

* At step "2" above, the client remains attached, basically escaping any
  check performed during command dispatch: read only slave, in that case.
* At step "5" the slave (that was the master), serves the blocked client
  consuming a list element, which is not consumed on the master side.

This scenario is technically likely to happen during failovers, however
since Redis Sentinel already disconnects clients using the CLIENT
command when changing the role of the instance, the bug is avoided in
Sentinel deployments.

Closes #2473.

Still many things to convert inside config.c in the next commits.
Some const safety in String objects creation and addReply() family
functions.

1. HVSTRLEN -> HSTRLEN. It's unlikely one needs the length of the key,
   not clear how the API would work (by value does not make sense) and
   there will be better names anyway.
2. Default is to return 0 when field is missing.
3. Default is to return 0 when key is missing.
4. The implementation was slower than needed, and produced unnecessary COW.

Related issue #2415.

Revert some size_t back to off_t
Set reply_bytes needs to 64 bits everywhere
Revert bufpos to int since it's a max of 16k into buf[]

the hvstrlen command returns the length of a hash field value

Severan problems are addressed but still a few missing.
Since replication of this command was more complex than others since it
needs to replicate multiple SREM commands, an old API able to do this
was reused (it was taken inside the implementation since it was pretty
obvious soon or later that would be useful). The API was improved a bit
so that now a command may opt-out for the standard command replication
when the server.dirty counter is incremented, in order to "manually"
replicate what it wants.

This also makes it backward compatible in the usage, but for the command
name. However the old command name was less obvious so it is worth to
break it probably.

With the new setup the program main can perform argument parsing and
everything else useful for an RDB check regardless of the Redis server
itself.

redis-check-dump is now named redis-check-rdb and it runs
as a mode of redis-server instead of an independent binary.

You can now use 'redis-server redis.conf --check-rdb' to check
the RDB defined in redis.conf.  Using argument --check-rdb
checks the RDB and exits.  We could potentially also allow
the server to continue starting if the RDB check succeeds.

This change also enables us to use RDB checking programatically
from inside Redis for certain failure conditions.

read() and write() return ssize_t (signed long), not int.

For other offsets, we can use the unsigned size_t type instead
of a signed offset (since our replication offsets and buffer
positions are never negative).

Also explicitly set version to 0, add a protocol version define, improve
comments in the gossip structure.

Note that the structure layout is the same after the change, we are just
making the padding explicit with an additional not used 16 bits field.
So this commit is still able to talk with the previous versions of
cluster nodes.

Adds configuration option 'supervised [no | upstart | systemd | auto]'

Also removed 'bzero' from the previous implementation because it's 2015.
(We could actually statically initialize those structs, but clang
throws an invalid warning when we try, so it looks bad even though it
isn't bad.)

Fixes #2264

This removes:
  - list-max-ziplist-entries
  - list-max-ziplist-value

This adds:
  - list-max-ziplist-size
  - list-compress-depth

Also updates config file with new sections and updates
tests to use quicklist settings instead of old list settings.

This replaces individual ziplist vs. linkedlist representations
for Redis list operations.

Big thanks for all the reviews and feedback from everybody in
https://github.com/antirez/redis/pull/2143

Previously, many files had individual main() functions for testing,
but each required being compiled with their own testing flags.
That gets difficult when you have 8 different flags you need
to set just to run all tests (plus, some test files required
other files to be compiled aaginst them, and it seems some didn't
build at all without including the rest of Redis).

Now all individual test main() funcions are renamed to a test
function for the file itself and one global REDIS_TEST define enables
testing across the entire codebase.

Tests can now be run with:
  - `./redis-server test <test>`

  e.g. ./redis-server test ziplist

If REDIS_TEST is not defined, then no tests get included and no
tests are included in the final redis-server binary.

Refactor a common pattern into one function so we don't
end up with copy/paste programming.