Use config cache via lcb_cntl() if possible

Change-Id: Ie0514e7da6a70ab36940f5049226d9c1470a0493
Reviewed-on: http://review.couchbase.org/36468
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>