CCBC-446: Undocument LCB_KV_CONTIG for keys

This is supported functionality but the API is a bit difficult to use.
Let's expose this once we have a cleaner way of doing it.

Change-Id: I9213459bdba3479719ed68e063c5a0eee684b961
Reviewed-on: http://review.couchbase.org/38405
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>