osd/scrub: move ReplicaReservations into the Scrubber FSM

osd/scrub: move ReplicaReservations into the Scrubber FSM

Handle grant/deny messages within the FSM.
One exception at this point: the handling of "granted by everyone"
(due to the technical inconvenience of having to handle the
"0 replicas" case in the FSM state constructor).

Note: after this commit, ScrubMachineListener - an API which is
a subset of the Scrubber API to be used by the Scrubber FSM - does
no longer make sense. The FSM should now have full access to the
scrubber, and that interface will be removed in a subsequent PR.

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