Add support for listen(2) backlog definition

In high RPS environments, the default listen backlog is not sufficient, so
giving users the power to configure it is the right approach, especially
since it requires only minor modifications to the code.
Option "backlog" renamed "tcp-backlog".

This is especially important since we already have a concept of backlog
(the replication backlog).
Redis.conf comment about tcp-backlog option improved.