Fixed:
- Do not determine `alive` status from the pidset returned
  by caller, since this can lead to "bouncing" state.
- Swap loop-check with sleep call, since sleep suspends
  the calling thread, which may lead to the callback
  being invoked if `alive` is invalidated by another
  thread.
- Do not trap INT and TERM by default; this was swallowing
  these signals for the Puma master.

Changelog: fixed