Adds a new function `should_process` to rooms impl that determines if
calling could should proceed with processing an event ID. Event IDs
are recorded (along with room ID) as a key pointing to the
system-local timestamp of when the event was received. If the key was
not originally present, we instruct calling code to process the event.
Events are also asychronously recorded by timestamp using a sled event
watcher that listens to inserts in the main tree (described above).
This secondary tree will allow easy cleanup of old events in the
future.
projectmoon