Sync engine: send(delay=) from a callback blocks the processing thread #706

Why

A 500 ms delayed send from an entry callback made the constructor of a sync machine take 0.5 s: the delay is waited inside the processing loop. Callers expect a delayed send to return immediately.

Scope

Confirm with a test on the sync engine, then decide whether delayed events should be scheduled without blocking or the limitation documented.

Found while measuring time events for #692. Refs #692