Timestamp shared memory messages on observation/receipt

[jkuszmaul]

Currently, messages sent on shared memory channels are timestamped prior to actually being "sent" (https://github.com/frc971/971-Robot-Code/blob/master/aos/ipc_lib/lockless_queue.cc#L1028). While we should have the guarantees in place to ensure that messages do not get sent with out-of-order timestamps on a given channel, the current ordering does mean that a process listening on multiple channels could plausibly observe messages across channels out-of-order (which isn't supposed to happen).

If we instead figured out an appropriate way to have the first observer of a message timestamp it (this "observer" would be the first of any fetchers or watchers, as well as something that would run immediately after the send actually went through), then because of how the event processing happens on the listening side, it should no longer be possible to observe out of order events.