osd/scrub: scheduling the next scrub following scrub completion

osd/scrub: scheduling the next scrub following scrub completion

or after an aborted scrub.

To note: one of the important changes in this commit:
merging the functionality of adjust_target_time() &
update_schedule() into a single function - adjust_schedule().

Regarding the handling of aborts:

Most of the time - all that is required following a scrub abort is to
requeue the scrub job - the one that triggered the aborted scrub -
with just a delay added to its n.b..

But we must take into account scenarios where "something" caused the
parameters prepared for the *next* scrub to show higher urgency or
priority. "Something" - as in an operator command requiring immediate
scrubbing, or a change in the pool/cluster configuration.
In such cases - the current requested flags and the parameters of
the aborted scrub must be merged.

Note that the current implementation is a temporary solution, to be
replaced by a per-level updating of the relevant target.

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