osd/scrub: decouple being reserved from handling scrub requests

osd/scrub: decouple being reserved from handling scrub requests

For a replica, following this change:

* 'ReplicaActive' captures the state of the scrubber when
  acting as a replica, from peering to interval change;

* "being reserved" is just a flag maintained by ReplicaActive, and
  is no longer a prerequisite for handling scrub requests.

* each scrub request is now associated with its own 'token' value.

and the following minor simplification:

* the 'should we wait for pushes' decision is now part of the
  code executed on the transition from ReplicaIdle into ReplicaActiveOp.
  StartReplicaNoWait can now be discarded.

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