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.

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.

Sending an ACK is now moved into the replicationSendAck() function.

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.

When master send commands, there is no need for the slave to reply.
Redis used to queue the reply in the output buffer and discard the reply
later, this is a waste of work and it is not clear why it was this way
(I sincerely don't remember).

This commit changes it in order to don't queue the reply at all.

All tests passing.

We don't write the output buffer to the client socket for slaves only if
the slave is not online.

Should be "continue" and was "return".

This fixes issue #1110

Required because of recent changes in the way logfile is set to standard
output.

Also the logfile option was modified to always have an explicit value
and to log to stdout when an empty string is used as log file.

Previously there was special handling of the string "stdout" that set
the logfile to NULL, this always required some special handling.

The test checks both successful syncs and unsuccessful ones by changing
the backlog size.

The test actually worked, but vars for master and slave were inverted
and sometimes used incorrectly.

bitcount commant may return negtive integer with string length more than 256 MB

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.