osd: introduce simple sleep during scrub

This option is similar to osd_snap_trim_sleep: simply inject an optional sleep
in the thread that is doing scrub work.  This is a very kludgey and coarse
knob for limiting the impact of scrub on the cluster, but can help until we
have a more robust and elegant solution.

Only sleep if we are in the NEW_CHUNK state to avoid delaying processing of an
in-progress chunk.  In this state nothing is blocked on anything. 
Conveniently, chunky_scrub() requeues itself for each new chunk.

Backport: firefly, dumpling Signed-off-by: Sage Weil <sage@inktank.com>