jqueue: Factorize code waiting for job changes

By splitting the _WaitForJobChangesHelper class into multiple smaller
classes, we gain in several places:

- Simpler code, less interaction between functions and variables
- Easy to unittest (close to 100% coverage)
- Waiting for job changes has no direct knowledge of queue anymore (it
  doesn't references queue functions anymore, especially not private ones)
- Activate inotify only if there was no change at the beginning (and
  checking again right away to avoid race conditions)

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>