osd: allow limited scrub preemption

osd: allow limited scrub preemption

If we receive a write within the scrub range, abort the scrub chunk and
shrink the chunk size.  If we do this too many times do not preempt and
allow the scrub to complete (to avoid scrub starvation due to client io).

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 6dd42392c0f00011059ffa5de74cace7d1e911bd)

Conflicts:
	src/messages/MOSDRepScrub.h
	src/messages/MOSDRepScrubMap.h
	src/osd/PrimaryLogPG.cc
	src/osd/PrimaryLogPG.h

- encode vs ::encode etc
- dragged in waiting for scrub events from 508ea640e3b
- ignore change in chunked manifest code (which dne in luminous)