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: Ieb328feaf25d544e06f76e6af8f44f84a72a519f
Reviewed-on: http://review.couchbase.org/36252
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>