Upgrade from 3.0.3 to 3.0.4.

<ChangeLog>

Upgrade urgency: HIGH for Redis and Sentinel. However note that in order to

fix certain replication bugs, the replication internals were
modified in a very heavy way. So while this release is
conceptually saner, it may contain regressions. For this
reason, before the release, QA activities were performed by
me (antirez) and Redis Labs and no evident bug was found.

• [FIX] A number of bugs related to replication PSYNC and the (yet experimental) diskless replication feature were fixed. The bugs could lead to inconsistency between masters and slaves. (Salvatore Sanfilippo, Oran Agra fixed the issue found by Yuval Inbar)
• [FIX] A replication bug in the context of PSYNC partial resynchonization was found and fixed. This bug happens even when diskless replication is off in the case different slaves connect at different times while the master is creating an RDB file, and later a partial resynchronization is attempted by a slave that connected not as the first one. (Salvatore Sanfilippo, Oran Agra)
• [FIX] Chained replication and PSYNC interactions leading to potential stale chained slaves data set, see issue #2694. (Salvatore Sanfilippo fixed an issue reported by "GeorgeBJ" user at Github)
• [FIX] redis-cli --scan iteration fixed when returned cursor overflows 32 bit signed integer. (Ofir Luzon, Yuval Inbar)
• [FIX] Senitnel: fixed a bug during the master switch process, where for a failed conditional check, the new configuration is rewritten, during a small window of time, in a corrupted way where the master is also reported to be one of the slaves. This bug is rare to trigger but apparently it happens in the wild, and the effect is to see a replication loop where the master will try to replicate with itself. A detailed explanation of the bug and its effects can be found in the commit message here: https://github.com/antirez/redis/commit/c20218eb5770b2cafb12bc7092313b8358fedc0a. The bug was found by Jan-Erik Rediger using a static analyzer and fixed by Salvatore Sanfilippo.
• [FIX] Sentinel lack of arity checks for certain commands. (Rogerio Goncalves, Salvatore Sanfilippo)

• [NEW] Replication internals rewritten in order to be more resistant to bugs. The replication handshake in the slave side was rewritten as a non blocking state machine. (Salvatore Sanfilippo, Oran Agra)
• [NEW] New "replication capabilities" feature introduced in order to signal from the master to the slave what are the features supported, so that the master can choose the kind of replication to start (diskless or not) when master and slave are of different versions. (Oran Agra, Salvatore Sanfilippo)
• [NEW] Log clients details when SLAVEOF command is received. (Salvatore Sanfilippo with inputs from Nick Craver and Marc Gravell).

</ChangeLog>