Replication: when possible start RDB saving ASAP.
Replication: when possible start RDB saving ASAP.

In a previous commit the replication code was changed in order to
centralize the BGSAVE for replication trigger in replicationCron(),
however after further testings, the 1 second delay imposed by this
change is not acceptable.

So now the BGSAVE is only delayed if the AOF rewriting process is
active. However past comments made sure that replicationCron() is always
able to trigger the BGSAVE when needed, making the code generally more
robust.

The new code is more similar to the initial @oranagra patch where the
BGSAVE was delayed only if an AOF rewrite was in progress.

Trivia: delaying the BGSAVE uncovered a minor Sentinel issue that is now
fixed.

xuguruogu/redis,xuguruogu/redis,pedigree/redis,4396/redis,4396/redis,xuguruogu/redis,pedigree/redis,xuguruogu/redis,4396/redis,4396/redis,pedigree/redis,pedigree/redis