osd/scrub: directly create or reinit the ScrubStore

osd/scrub: directly create or reinit the ScrubStore

The ScrubStore is now directly created or reinitialized by the
Scrubber. Note that the store object is not identical to the
errors DB: the errors DB is an entity in the OSD store (a
collection of OMap entries in a uniquely-named object(s)),
while the ScrubSTore object is a cacher and accessor for
that entity. That one can be recreated or disposed of at
will.

We now do not recreate the ScrubStore object for every scrub.

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