osd/scrub: reserve replicas one by one, and in consistent order

osd/scrub: reserve replicas one by one, and in consistent order

Issuing the reservation requests one by one - waiting for
approval from the secondary before the next request is sent.

The requests are sent in ascending target pg-shard-id order, reducing the
chance of having two PGs repeatedly competing for the same set of
OSDs - and doing so in an interleaved sequence.

Modifying the Session state in the scrubber FSM to react to interval
changes by discarding replica reservations.

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