Rework instance <-> cmdq relationship

This commit properly assigns the instance to the command queue via an
opaque 'cqdata' field. The field has been renamed from instance to
indicate the separation of concerns between the mc and lcb modules.

This commit also ensures the cqdata field is assigned during inital
instance creation, rather than each time we get a config

Change-Id: I6672550127d87a3436a9e30b02cbac2d1a8d300f
Reviewed-on: http://review.couchbase.org/39820
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>