libvbucket: parse 'rev' field

This adds a vbucket_config_get_revision() field which returns the
revision ID (if parsed) or -1 (if not found).

Change-Id: I1de653cf4823db0f914eee4964d003c0da4eb80e
Reviewed-on: http://review.couchbase.org/36301
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>