lcb_cntl_string: understand 'compression=force'

This is useful for debugging and with commands like cbc where a user
desires to store only a single value.

Change-Id: I38c2828329583118922eb5d6211201ec4ff02af4
Reviewed-on: http://review.couchbase.org/38090
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>