osd: Add config option to disable new scrubs during recovery

osd: Add config option to disable new scrubs during recovery

The osd_scrub_during_recovery config option allows for configuring
if the OSD will schedule a new scrub while recovery is active.

When set to false no new scrubs will be initiated by the OSD while
there are recovery threads active on that OSD.

Signed-off-by: Wido den Hollander <wido@42on.com>
(cherry picked from commit 33f1f6077804dbcbb82a41ef7f6803b5f3365831)

Conflicts:
	src/osd/OSD.h (between jewel and kraken, pg recovery methods were moved
        from the OSD class to OSDService)