osd/scrub: the scrub queue now holds a copy of the ScrubJob

osd/scrub: the scrub queue now holds a copy of the ScrubJob

... and not a shared pointer to it.
The PG's entry in the scrub queue and the scrub-job object that
is part of the scrubber are now separate entities, that can be
modified separately - each with its own locking mechanism.

the interaction between OsdScrub (the OSD's scrub scheduler
object) and the scrub queue during scrub initiation is simplified:
instead of fetching from the queue a list of scrub targets
possibly ready to be scrubbed, the OsdScrub now picks only one
candidate - the top of the queue.

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