SENTINEL SET master quorum implemented.
Set REDIS_AOF_REWRITE_MIN_SIZE to 64mb.

64mb is the default value in redis.conf. For some reason instead the
hard-coded default was 1mb that is too small.
Fix typo in aofRewriteBufferAppend() comment.
Cluster: support to read from slave nodes.

A client can enter a special cluster read-only mode using the READONLY
command: if the client read from a slave instance after this command,
for slots that are actually served by the instance's master, the queries
will be processed without redirection, allowing clients to read from
slaves (but without any kind fo read-after-write guarantee).

The READWRITE command can be used in order to exit the readonly state.