HTTP: Adapt to lcbio_CTX-based buffers

Change-Id: I638e43765f103477a1e4c8f62c73159802bc6400
Reviewed-on: http://review.couchbase.org/35882
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>