CCBC-442: Allow string key=value mode for lcb_cntl()

Change-Id: I3c18bfe4ec79bf249f36cbfef6083cac38ad1ab4
Reviewed-on: http://review.couchbase.org/37788
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>