bc_http: Adapt to lcbio_CTX-buffers

Change-Id: If6c396b3c98abd6ba7ded5305415114239734b6c
Reviewed-on: http://review.couchbase.org/35883
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>