osd/scrub: modify ScrubJob to hold two SchedTarget-s

osd/scrub: modify ScrubJob to hold two SchedTarget-s

ScrubJob will now hold two SchedTarget-s - two sets of scheduling
information (times, levels, etc.) for the next shallow and deep scrubs.

This is in preparation for the upcoming changes to the scheduling queue.
The change cannot stand on its own, as the partial implementation
creates some inconsistencies in the scheduling logic.

Specifically, here is what changes here, and how it differs from the
desired implementation:
- The OSD still maintains a queue of scrub jobs - one object only per
  PG.
  But now - each queue element holds two SchedTarget-s.
- When a scrub is initiated, the Scrubber is handed a ScrubJob object.
  Only in the next commit will it also receive the ID of the selected
  level. That causes some issues when re-determining the level of the
  initiated scrub. A failure to match the queue "intent" results in
  failures.
- the 'planned scrub' flags are still here, instead of directly
  encoding the characteristics of the next scrub in the relevant
  sched-entry.
- the 'urgency' levels do not cover the full required range of
  behaviors and priorities.

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