Define 'transfer' field for protoctx

This allows the protocol context itself to assign itself to the
connection object, with a hook to handle scenarios where the underlying
storage for the connection has changed

Change-Id: I850f574651c302d67586a1e0eedd9c6ed1292e2a
Reviewed-on: http://review.couchbase.org/34592
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>