=per #15575 lessen timing issue in AtLeastOnce spec

* lower redelivery interval does not change semantics here (no one
  answers anyway), and lessens the changes to be "unlucky" with gc pauses
* was unable to reproduce error with snapshotting, incl. benching / 80builds on jenkins...
* enabled debug logging for this test, so we have more clues next time it fails, there's a todo to remove the debug messages - will do so after happy with it's stability

Resolves #15575