- Remove `removeAllOutstandingCallbacks` method. This never had any
  usage outside of `scheduled_test` which has been deprecated for a long
  time.
- Change some bare `Future` to `Future<void>`.
- Move and rename `OutstandingCallbackCounter` class - the
  implementation has no ties to the concept of callbacks, but that is
  how it is used.
- Use the modern syntax for function typed arguments.
- Add generics to `unclosable` to void throwing away potentially useful
  type information.