REPLCONF ACK command.

This special command is used by the slave to inform the master the
amount of replication stream it currently consumed.

it does not return anything so that we not need to consume additional
bandwidth needed by the master to reply something.

The master can do a number of things knowing the amount of stream
processed, such as understanding the "lag" in bytes of the slave, verify
if a given command was already processed by the slave, and so forth.
Replication: send REPLCONF ACK to master.
Make sure that REPLCONF ACK really has no return value.
Don't ACK the master after every command.

Sending an ACK is now moved into the replicationSendAck() function.
Send ACK to master once every second.

ACKs can be also used as a base for synchronous replication. However in
that case they'll be explicitly requested by the master when the client
sends a request that needs to be replicated synchronously.
Close connection with timedout slaves.

Now masters, using the time at which the last REPLCONF ACK was received,
are able to explicitly disconnect slaves that are no longer responding.

Previously the only chance was to see a very long output buffer, that
was highly suboptimal.
redis.conf updated: repl-timeout now uesd by masters as well.
Version bumped to 2.9.11
A comment about BLPOP timeout did not reflected actual behavior.
Merge remote-tracking branch 'redis/unstable' into sentinel-config-rewrite