osd: implement scrub_purged_snaps command

osd: implement scrub_purged_snaps command

This a naive one-shot implementation that does the full scan synchronously
in the command thread.  It shouldn't block any IO except to the extent
that it will compete for IO reading the underlying snapmapper omap object.

When we discover mapped objects that are covered by ranges of snaps that
should be purged, we requeue the snapid for trim on the relevant PG(s).
For these 'repeat' trims we skip the final step(s) to mark the snapid as
purged, since that presumably already happened some time ago.

Signed-off-by: Sage Weil <sage@redhat.com>