osd/scrub: passing the scrub-job copy through the scrubber

osd/scrub: passing the scrub-job copy through the scrubber

moving the scrubbed sjob copy thru the scrubber - from scrub
session initiation to its termination (or abort - where we use the
handed "old version" of the sjob to update the new one).

Note that in this version - not all the information that was used to
determine the specifics of the initiated scrub is passed to the
scrubber and back. In this half-baked stage of the refactoring,
the resulting implementation handling of corner cases, still using
the "planned scrub" flags, is less than optimal.

The next step (dual targets, replacing the 'planned scrub' flags with
specific attributes in the scheduling target) fixes this.

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