Sentinel: SENTINEL FAILOVER command implemented.

This command can be used in order to force a Sentinel instance to start
a failover for the specified master, as leader, forcing the failover
even if the master is up.

The commit also adds some minor refactoring and other improvements to
functions already implemented that make them able to work when the
master is not in SDOWN condition. For instance slave selection
assumed that we ask INFO every second to every slave, this is true
only when the master is in SDOWN condition, so slave selection did not
worked when the master was not in SDOWN condition.
Merge branch 'unstable' of git://github.com/antirez/redis into unstable
Add ziplist test for deleting next to last entries
Set p to its new offset before modifying it
hiredis library updated.

This version of hiredis merges modifications of the Redis fork with
latest changes in the hiredis repository.

The same version was pushed on the hiredis repository and will probably
merged into the master branch in short time.
redis-benchmark: disable big buffer cleanup in hiredis context.

This new hiredis features allows us to reuse a previous context reader
buffer even if already very big in order to maximize performances with
big payloads (Usually hiredis re-creates buffers when they are too big
and unused in order to save memory).
Merge pull request #628 from pietern/unstable-zip

Fix ziplist edge case
Merge branch 'unstable' of git://github.com/antirez/redis into unstable
Sentinel: fixed a crash on script execution.

The call to sentinelScheduleScriptExecution() lacked the final NULL
argument to signal the end of arguments. This resulted into a crash.
Sentinel: send SCRIPT KILL on -BUSY reply and SDOWN instance.

From the point of view of Redis an instance replying -BUSY is down,
since it is effectively not able to reply to user requests. However
a looping script is a recoverable condition in Redis if the script still
did not performed any write to the dataset. In that case performing a
fail over is not optimal, so Sentinel now tries to restore the normal server
condition killing the script with a SCRIPT KILL command.

If the script already performed some write before entering an infinite
(or long enough to timeout) loop, SCRIPT KILL will not work and the
fail over will be triggered anyway.
Better Out of Memory handling.

The previous implementation of zmalloc.c was not able to handle out of
memory in an application-specific way. It just logged an error on
standard error, and aborted.

The result was that in the case of an actual out of memory in Redis
where malloc returned NULL (In Linux this actually happens under
specific overcommit policy settings and/or with no or little swap
configured) the error was not properly logged in the Redis log.

This commit fixes this problem, fixing issue #509.
Now the out of memory is properly reported in the Redis log and a stack
trace is generated.

The approach used is to provide a configurable out of memory handler
to zmalloc (otherwise the default one logging the event on the
standard output is used).
Fix a forget zmalloc_oom() -> zmalloc_oom_handler() replacement.
Incrementally flush RDB on disk while loading it from a master.

This fixes issue #539.

Basically if there is enough free memory the OS may buffer the RDB file
that the slave transfers on disk from the master. The file may
actually be flused on disk at once by the operating system when it gets
closed by Redis, causing the close system call to block for a long time.

This patch is a modified version of one provided by yoav-steinberg of
@garantiadata (the original version was posted in the issue #539
comments), and tries to flush the OS buffers incrementally (every 8 MB
of loaded data).
Sentinel: suppress harmless warning by initializing 'table' to NULL.

Note that the assertion guarantees that one of the if branches setting
table is always entered.
Sentinel: Redis-side support for slave priority.

A Redis slave can now be configured with a priority, that is an integer
number that is shown in INFO output and can be get and set using the
redis.conf file or the CONFIG GET/SET command.

This field is used by Sentinel during slave election. A slave with lower
priority is preferred. A slave with priority zero is never elected (and
is considered to be impossible to elect even if it is the only slave
available).

A next commit will add support in the Sentinel side as well.
Sentinel: Sentinel-side support for slave priority.

The slave priority that is now published by Redis in INFO output is
now used by Sentinel in order to select the slave with minimum priority
for promotion, and in order to consider slaves with priority set to 0 as
not able to play the role of master (they will never be promoted by
Sentinel).

The "slave-priority" field is now one of the fileds that Sentinel
publishes when describing an instance via the SENTINEL commands such as
"SENTINEL slaves mastername".
Sentinel: added documentation about slave-priority in redis.conf
Sentinel: add Redis execution mode to INFO output.

The new "redis_mode" field in the INFO output will show if Redis is
running in standalone mode, cluster, or sentinel mode.
Sentinel: INFO command implementation.
Merge branch 'unstable' of git://github.com/antirez/redis into unstable