Use macro for vbucket config handle

This will allow us to more smoothly transition between later refactors
of the mc_CMDQUEUE object where the vbucket config is no longer part of
lcb_t itself. Only subsystems which are intended to remain relatively
untouched have been given the new macro usage; whereas systems which
will be removed or rewritten will continue to use the 'old' variant.

Change-Id: I56817267c98c36b8c096337b9912859b3644aeda
Reviewed-on: http://review.couchbase.org/34595
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>