osd/scrub: OSD's scrub queue now holds SchedEntry-s

osd/scrub: OSD's scrub queue now holds SchedEntry-s

The OSD's scrub queue now holds SchedEntry-s, instead of ScrubJob-s.
The queue itself is implemented using the 'not_before_queue_t' class.

Note: this is not a stable state of the scrubber code. In the next
commits:
- modifying the way sched targets are modified and updated, to match the
  new queue implementation.
- removing the 'planned scrub' flags.

Important note: the interaction of initiate_scrub() and pop_ready_pg()
is not changed by this commit. Namely:

Currently - pop..() loops over all eligible jobs, until it finds one
that matches the environment restrictions (which most of the time, as the
concurrency limit is usually reached, would be 'high-priority-only').

The other option is to maintain Sam's 'not_before_q' clean interface: we
always pop the top, and if that top fails the preconds tests - we delay and
re-push. This has the following troubling implications:

- it would take a long time to find a viable scrub job, if the problem
  is related to, for example, 'no scrub'.
- local resources failure (inc_scrubs() failure) must be handles
  separately, as we do not want to reshuffle the queue for this
  very very common case.
- but the real problem: unneeded shuffling of the queue, even as the
  problem is not with the scrub job itself, but with the environment
  (esp. no-scrub etc.).
  This is a common case, and it would be wrong to reshuffle the queue
  for that.
- and - remember that any change to a sched-entry must be done under PG
  lock.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>