PING can now be called with an additional arugment, behaving exactly
like the ECHO command. PING can now also be called in Pub/Sub mode (with
one more more subscriptions to channels / patterns) in order to trigger
the delivery of an asynchronous pong message with the optional payload.

This fixes issue #420.

The code tested many times if a client had active Pub/Sub subscriptions
by checking the length of a list and dictionary where the patterns and
channels are stored. This was substituted with a client flag called
REDIS_PUBSUB that is simpler to test for. Moreover in order to manage
this flag some code was refactored.

This commit is believed to have no effects in the behavior of the
server.

Previously, the command definition for the OBJECT command specified
a minimum of two args (and that it was variadic), which meant that
if you sent this:

OBJECT foo

When cluster was enabled, it would result in an assertion/SEGFAULT
when Redis was attempting to extract keys.

It appears that OBJECT is not variadic, and only ever takes 3 args.

https://gist.github.com/michael-grunder/25960ce1508396d0d36a

We introduce the distinction between slow and fast commands since those
are two different sources of latency. An O(1) or O(log N) command without
side effects (can't trigger deletion of large objects as a side effect of
its execution) if delayed is a symptom of inherent latency of the system.

A non-fast command (commands that may run large O(N) computations) if
delayed may just mean that the user is executing slow operations.

The advices LATENCY should provide in this two different cases are
different, so we log the two classes of commands in a separated way.

This fixes detection of wrong subcommand (that resulted in the default
all-commands output instead) and allows COMMAND INFO to be called
without arguments (resulting into an empty array) which is useful in
programmtically generated calls like the following (in Ruby):

    redis.commands("command","info",*mycommands)

Note: mycommands may be empty.

Static was removed since it is needed in order to get symbols in stack
traces. Minor changes in the source code were operated to make it more
similar to the existing Redis code base.

COMMANDS returns a nested multibulk reply for each
command in the command table.  The reply for each
command contains:
  - command name
  - arity
  - array of command flags
  - start key position
  - end key position
  - key offset step
  - optional: if the keys are not deterministic and
    Redis uses an internal key evaluation function,
    the 6th field appears and is defined as a status
    reply of: REQUIRES ARGUMENT PARSING

Cluster clients need to know where the keys are in each
command to implement proper routing to cluster nodes.

Redis commands can have multiple keys, keys at offset steps, or other
issues where you can't always assume the first element after
the command name is the cluster routing key.

Using the information exposed by COMMANDS, client implementations
can have live, accurate key extraction details for all commands.

Also implements COMMANDS INFO [commands...] to return only a
specific set of commands instead of all 160+ commands live in Redis.

Fixes #1826 (and many other reports of the same problem)

This will be used by CLIENT KILL and is also a good way to ensure a
given client is still the same across CLIENT LIST calls.

The output of CLIENT LIST was modified to include the new ID, but this
change is considered to be backward compatible as the API does not imply
you can do positional parsing, since each filed as a different name.

Because of output buffer limits Redis internals had this idea of type of
clients: normal, pubsub, slave. It is possible to set different output
buffer limits for the three kinds of clients.

However all the macros and API were named after output buffer limit
classes, while the idea of a client type is a generic one that can be
reused.

This commit does two things:

1) Rename the API and defines with more general names.
2) Change the class of clients executing the MONITOR command from "slave"
   to "normal".

"2" is a good idea because you want to have very special settings for
slaves, that are not a good idea for MONITOR clients that are instead
normal clients even if they are conceptually slave-alike (since it is a
push protocol).

The backward-compatibility breakage resulting from "2" is considered to
be minimal to care, since MONITOR is a debugging command, and because
anyway this change is not going to break the format or the behavior, but
just when a connection is closed on big output buffer issues.

The new ROLE command is designed in order to provide a client with
informations about the replication in a fast and easy to use way
compared to the INFO command where the same information is also
available.

Replication is totally broken when a slave has this option, since it
stops accepting updates from masters.

This fixes issue #1434.

When the listening sockets readable event is fired, we have the chance
to accept multiple clients instead of accepting a single one. This makes
Redis more responsive when there is a mass-connect event (for example
after the server startup), and in workloads where a connect-disconnect
pattern is used often, so that multiple clients are waiting to be
accepted continuously.

As a side effect, this commit makes the LOADING, BUSY, and similar
errors much faster to deliver to the client, making Redis more
responsive when there is to return errors to inform the clients that the
server is blocked in an not interruptible operation.

Like ZCOUNT for lexicographical ranges.

It is safer since it is able to have side effects.

PFDEBUG will be the interface to do debugging tasks with a key
containing an HLL object.

adjustOpenFilesLimit() and clusterUpdateSlotsWithConfig() that were
assuming uint64_t is the same as unsigned long long, which is true
probably for all the systems out there that we target, but still GCC
emitted a warning since technically they are two different types.

The new command allows to get a dump of the registers stored
into an HyperLogLog data structure for testing / debugging purposes.

Using both the initials of Philippe Flajolet instead of just "P".

Merge N HLL data structures by selecting the max value for every
M[i] register among the set of HLLs.

To test the bitfield array of counters set/get macros from the Redis Tcl
suite is hard, so a specialized command that is able to test the
internals was developed.

In this commit:
* Decrement steps are semantically differentiated from the reserved FDs.
  Previously both values were 32 but the meaning was different.
* Make it clear that we save setrlimit errno.
* Don't explicitly handle wrapping of 'f', but prevent it from
  happening.
* Add comments to make the function flow more readable.

This integrates PR #1630