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

We have a check to rewrite the config properly when a failover is in
progress, in order to add the current (already failed over) master as
slave, and don't include in the slave list the promoted slave itself.

However there was an issue, the variable with the right address was
computed but never used when the code was modified, and no tests are
available for this feature for two reasons:

1. The Sentinel unit test currently does not test Sentinel ability to
persist its state at all.
2. It is a very hard to trigger state since it lasts for little time in
the context of the testing framework.

However this feature should be covered in the test in some way.

The bug was found by @badboy using the clang static analyzer.

Effects of the bug on safety of Sentinel
===

This bug results in severe issues in the following case:

1. A Sentinel is elected leader.
2. During the failover, it persists a wrong config with a known-slave
entry listing the master address.
3. The Sentinel crashes and restarts, reading invalid configuration from
disk.
4. It sees that the slave now does not obey the logical configuration
(should replicate from the current master), so it sends a SLAVEOF
command to the master (since the slave master is the same) creating a
replication loop (attempt to replicate from itself) which Redis is
currently unable to detect.
5. This means that the master is no longer available because of the bug.

However the lack of availability should be only transient (at least
in my tests, but other states could be possible where the problem
is not recovered automatically) because:

6. Sentinels treat masters reporting to be slaves as failing.
7. A new failover is triggered, and a slave is promoted to master.

Bug lifetime
===

The bug is there forever. Commit 16237d78 actually tried to fix the bug
but in the wrong way (the computed variable was never used! My fault).
So this bug is there basically since the start of Sentinel.

Since the bug is hard to trigger, I remember little reports matching
this condition, but I remember at least a few. Also in automated tests
where instances were stopped and restarted multiple times automatically
I remember hitting this issue, however I was not able to reproduce nor
to determine with the information I had at the time what was causing the
issue.

Update t_zset.c

We usually want to reach the master using the address of the interface
Redis is bound to (via the "bind" config option). That's useful since
the master will get (and publish) the slave address getting the peer
name of the incoming socket connection from the slave.

However, when this is not possible, for example because the slave is
bound to the loopback interface but repliaces from a master accessed via
an external interface, we want to still connect with the master even
from a different interface: in this case it is not really important that
the master will provide any other address, while it is vital to be able
to replicate correctly.

Related to issues #2609 and #2612.

This performs a best effort source address binding attempt. If it is
possible to bind the local address and still have a successful
connect(), then this socket is returned. Otherwise the call is retried
without source address binding attempt.

Related to issues #2609 and #2612.

Two code paths jumped to the "ok, return the socket to the user" code
path to handle error conditions.

Related to issues #2609 and #2612.

Related to issues #2609 and #2612.

From Twitter:

    "@antirez that’s an awfully-named command :(
     http://en.wikipedia.org/wiki/Retching"

Normally ZADD only returns the number of elements added to a sorted
set, using the RETCH option it returns the sum of elements added or
for which the score was updated.

Update anet.c

Removed incorrect suggestion

adding a sentinel command: "flushconfig" per RCP4

This commit adds the SENTINEL simulate-failure, that sets specific
hooks inside the state machine that will crash Sentinel, for testing
purposes.

DEL/INCR/DECR and others could be NTH but apparently never made it to the implementation of SORT

Trivial omission of the obvious no-match case.

A way for monitoring systems to check that Sentinel is technically able
to reach the quorum and failover, using the currently visible Sentinels.

Issue #2517, smove contract violation

protocol error log should be seen by debug/verbose level