Ceph

Sage wrote, as an initial comment of the pull request:

This will make hammer users' lives easier by avoiding the bunched-up scrub times. (Just talked to another big user today who has to disable automatic scrubbing and schedule manually w/ cron.)

There was one conflict due to the missing osd_lock.is_locked() assert in
sched_scrub (after hammer the locking changes). I don't think it'll cause
a problem here...