osd: Move PG::_scan_list() to backend as ReplicatedBackend::be_scan_list()

Signed-off-by: David Zafman <david.zafman@inktank.com>
osd: Move PG::_scan_snaps() to backend as ReplicatedBackend::be_scan_snaps()

Signed-off-by: David Zafman <david.zafman@inktank.com>
osd: Move functions in order to see required changes

This requires the commit that follows (does not build)
Move function ReplicatedPG::_scrub()
Move struct scrubber to osd_types.h

Signed-off-by: David Zafman <david.zafman@inktank.com>
Debugging this