Switch protocol to using serial numbers for ordering events and requests
The wayland protocol, as X, uses timestamps to match up certain requests with input events. The problem is that sometimes we need to send out an event that doesn't have a corresponding timestamped input event. For example, the pointer focus surface goes away and new surface needs to receive a pointer enter event. These events are normally timestamped with the evdev event timestamp, but in this case, we don't have a evdev timestamp. So we have to go to gettimeofday (or clock_gettime()) and then we don't know if it's coming from the same time source etc. However for all these cases we don't need a real time timestamp, we just need a serial number that encodes the order of events inside the server. So we introduce a serial number mechanism that we can use to order events. We still need real-time timestamps for actual input device events (motion, buttons, keys, touch), to be able to reason about double-click speed and movement speed so events that correspond to user input carry both a serial number and a timestamp. The serial number also give us a mechanism to key together events that are "logically the same" such as a unicode event and a keycode event, or a motion event and a relative event from a raw device.