Adjust the `tractor._spawn.soft_wait()` strategy to avoid sending an
actor cancel request (via `Portal.cancel_actor()`) if either the child
process is detected as having terminated or the IPC channel is detected
to be closed.

This ensures (even) more deterministic inter-actor cancellation by
avoiding the timeout condition where possible when a whild never
sucessfully spawned, crashed, or became un-contactable over IPC.