Code review comment for lp:~raof/mir/prebump-abi-for-lifecycle-cookie

The idea here is that sometimes clients will need to refer to a past event. They will do this with a MirEventCookie. This is generally useful - for example, we could use it in the modesetting code to verify that the configuration the client is trying to set is newer than the last hardware hotplug event, and if we tag each input event with a cookie it's useful for properly handing off focus to new surfaces/clients.

In this particular case, I'll later add a mir_connection_lifecycle_save_complete(MirConn, MirEventCookie), so that clients can tell Mir when they've finished handling the mir_lifecycle_state_may_suspend event.

I just wanted to land this while we hadn't yet transitioned to libmirclient3, to save some work.