osd/scrub: modify "classic" OSD scrub state-machine to support Crimson

osd/scrub: modify "classic" OSD scrub state-machine to support Crimson

As some scrub-related functions are asynchronous in Crimson,
scrub states that call those functions cannot simply perform a
'post' or state-transition sequentially. The called operations
must arrange for a state-machine event to be sent upon completion.

Specifically, the following are now handled (on the FSM side) as async:
 - building scrub maps
 - comparing the scrub maps (and the rest of "what we
   do after a chunk is  handled")

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