4139: Node Manager callback actor avoids redundant delivery calls.

This helps make it more responsive when it receives a shutdown
request.
4139: Improve logging in Node Manager poll actors.
4139: Improve scheduling of Node Manager polls.

* Catch all exceptions, so that we keep polling no matter what
  happens.  Use CLIENT_ERRORS as a hint about how much logging we
  need.
* Make the next poll time calculation a little less stateful and
  easier to follow.
* Add tests for poll scheduling.
Merge branch '4139-node-manager-polling-logging-wip'

Refs #4275, #4139.