luminous: osd: Implement lazy omap usage statistics

luminous: osd: Implement lazy omap usage statistics

Opportunistic gathering of omap statistics during deep scrub.

Signed-off-by: Brad Hubbard <bhubbard@redhat.com>
(cherry picked from commit 9e21ef0e40ff89b10dfdf857c703336496b3171f)

 Conflicts:
	doc/dev/placement-group.rst: Trivial
	src/osd/PGBackend.cc: Small difference in ScrubMap::object init
	src/osd/osd_types.cc: Differences in structure versions and
        members
	src/osd/osd_types.h: Differences in structure versions and
        members