Adjust the `soft_wait()` strategy to avoid sending needless cancel
requests if it is known that a child process is already terminated or
does so before the cancel request times out. This should be no slower
and should avoid needless waits on either closure-in-progress or already
closed channels.
Basic strategy is,
- request child actor to cancel
- if process termination is detected, cancel the cancel
- if the process is still alive after a cancel request timeout warn the
user and yield back to the hard reap handling