crimson/osd: handle MOSDScrub2

crimson/osd: handle MOSDScrub2

MOSDScrub2 is sent from mgr for serving "ceph pg
{scrub|deep-scrub|repair}' commands when it's talking to a mimic and newer OSD.

ceph task checks if all pgs are scrubbed by looking at the `last_scrub_stamp` fields
in the `ceph pg dump` output. and it request the not-yet-scrubbed pgs a
deep scrub to ensure they are scrub before timeout.

in this change, crimson handles MOSDScrub2 by starting a remote peering
request, and the underlying peering_state will notify the corresponding
PG to start scrub. to get the test pass, a minimal implmentation is
added to update the scrub timestamp to `now` upon request of
peering_state.

we will need to add the correct scrubbing support later. but this is
enough for passing the thrasher test and for preparing for more tests
which uses the "ceph" task.

Signed-off-by: Kefu Chai <kchai@redhat.com>