Utilize connection pool for memcached sockets

This eliminates extra connections for memcached when using CCCP sockets.
In other words, CCCP no longer allocates its own "Private" socket, but
rather releases its private connection to the pool once it has completed
using it.

Additionally, cccp will now use existing memcached sockets for cluster
information (if so required) rather than creating new sockets from the
pool

Change-Id: I9c8fe908248d96103672915474f048720e1b9e3d
Reviewed-on: http://review.couchbase.org/32883
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>