MC Packet/Request Subsystem

This adds the mc subsytem used for tracking packets and writing them to
the network. MC comes with its own tests.

Change-Id: I29d59df7ab8825eb225c2a2804951b6476b35198
Reviewed-on: http://review.couchbase.org/34565
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>