osd/scrub: decoupling the FSM from the scrubber

osd/scrub: decoupling the FSM from the scrubber

The scrubber FSM has always been accessing the PgScrubber
object via an abstract interface (ScrubMachineListener).
But the reverse was not true: the PgScrubber owned a unique_ptr
directly to an instance of Scrubber::ScrubMachine.

This PR introduces a new interface, ScrubFsmIf, that is
implemented by the ScrubMachine. This simplifies
unit-testing the PgScrubber, as we can now inject a mock
implementation of the FSM interface.

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