Lua updated to version 5.1.5.
Sentinel: only demote old master into slave under certain conditions.

We used to always turn a master into a slave if the DEMOTE flag was set,
as this was a resurrecting master instance.

However the following race condition is possible for a Sentinel that
got partitioned or internal issues (tilt mode), and was not able to
refresh the state in the meantime:

1) Sentinel X is running, master is instance "A".
3) "A" fails, sentinels will promote slave "B" as master.
2) Sentinel X goes down because of a network partition.
4) "A" returns available, Sentinels will demote it as a slave.
5) "B" fails, other Sentinels will promote slave "A" as master.
6) At this point Sentinel X comes back.

When "X" comes back he thinks that:

"B" is the master.
"A" is the slave to demote.

We want to avoid that Sentinel "X" will demote "A" into a slave.
We also want that Sentinel "X" will detect that the conditions changed
and will reconfigure itself to monitor the right master.

There are two main ways for the Sentinel to reconfigure itself after
this event:

1) If "B" is reachable and already configured as a slave by other
sentinels, "X" will perform a redirection to "A".
2) If there are not the conditions to demote "A", the fact that "A"
reports to be a master will trigger a failover detection in "X", that
will end into a reconfiguraiton to monitor "A".

However if the Sentinel was not reachable, its state may not be updated,
so in case it titled, or was partiitoned from the master instance of the
slave to demote, the new implementation waits some time (enough to
guarantee we can detect the new INFO, and new DOWN conditions).

If after some time still there are not the right condiitons to demote
the instance, the DEMOTE flag is cleared.
Sentinel: more sensible delay in master demote after tilt.
Sentinel: changes to tilt mode.

Tilt mode was too aggressive (not processing INFO output), this
resulted in a few problems:

1) Redirections were not followed when in tilt mode. This opened a
   window to misinform clients about the current master when a Sentinel
   was in tilt mode and a fail over happened during the time it was not
   able to update the state.

2) It was possible for a Sentinel exiting tilt mode to detect a false
   fail over start, if a slave rebooted with a wrong configuration
   about at the same time. This used to happen since in tilt mode we
   lose the information that the runid changed (reboot).

   Now instead the Sentinel in tilt mode will still remove the instance
   from the list of slaves if it changes state AND runid at the same
   time.

Both are edge conditions but the changes should overall improve the
reliability of Sentinel.
Fix AOF bug: expire could be removed from key on AOF rewrite.

There was a race condition in the AOF rewrite code that, with bad enough
timing, could cause a volatile key just about to expire to be turned
into a non-volatile key. The bug was never reported to cause actualy
issues, but was found analytically by an user in the Redis mailing list:

https://groups.google.com/forum/?fromgroups=#!topic/redis-db/Kvh2FAGK4Uk

This commit fixes issue #1079.
Reset masterauth if an empty string is configured.
CONFIG SET server.masterauth aesthetic change.

This is just to make the code exactly like the above instance used for
requirepass. No actual change nor the original code violated the Redis
coding style.
Added more info about 32 bit builds into README.
in 32bit machine, popcount don't work with a input string length up to 512 MB,
bitcount commant may return negtive integer with string length more than 256 MB
Test: various issues with the replication-4.tcl test fixed.

The test actually worked, but vars for master and slave were inverted
and sometimes used incorrectly.
Test: check that replication partial sync works if we break the link.

The test checks both successful syncs and unsuccessful ones by changing
the backlog size.
Version bumped to 2.7.1
Test: more PSYNC tests (backlog TTL).
redis-cli: help.h updated.
Obtain absoute path of configuration file, expose it in INFO.
CONFIG REWRITE: Initial support code and design.
CONFIG REWRITE: support for save and syslog-facility.
CONFIG REWRITE: support for dir and slaveof.
CONFIG REWRITE: support for client-output-buffer-limit.
CONFIG REWRITE: strip multiple empty lines.