mon/PGMonitor: avoid iterating over all pgs to find stale

mon/PGMonitor: avoid iterating over all pgs to find stale

Instead of iterating over all pgs when an osd goes down, make a
set of all osds that might have gone down, and only check pgs that
it manages.  This is more efficient, especially for large clusters
with large numbers of OSDs.

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