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.
Test: fixed osx msg passing issue in testing framework.

The Redis test uses a server-clients model in order to parallelize the
execution of different tests. However in recent versions of osx not
setting the channel to a binary encoding caused issues even if AFAIK no
binary data is really sent via this channel. However now the channels
are deliberately set to a binary encoding and this solves the issue.

The exact issue was the test not terminating and giving the impression
of running forever, since test clients or servers were unable to
exchange the messages to continue.