P0.5: Watchdog auto delete leak priorityclass

[yqwang-ms]

Short summary about the issue/question:
Even after #3808 (comment), framework cannot be foreground deleted sometimes. (Behaivor changed for new version see #4117)
So, we should not continue to cross-namespace owner references anymore (even if blockOwnerDeletion=false), since it is disallowed by design.

So, in short term, we can periodically check if a priorityclass still has its corresponding framework, if not, watchdog should treat it as leak, and delete it automatically.

BTW, remove the owner references in RestServer, and delete all priorityclass when delete the FC.

In long term, we should customize queuing order by scheduler framework, instead of per job priority class object.