MB-9307: Handle rollback properly in fake UPR server

Change-Id: Ie65ee4f36d220deba979b673f5d15fc59e39b80b
Reviewed-on: http://review.couchbase.org/31298
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>