Top comment for prepareClientToWrite() clarified.

We don't write the output buffer to the client socket for slaves only if
the slave is not online.
Replication: don't even queue replies to master commands.

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.
Fixed a bug in no queueing replies to master.