osd/PeeringState: track down OSDs relevant to prior_readable_until_ub

osd/PeeringState: track down OSDs relevant to prior_readable_until_ub

Keep track of which OSDs from the prior set we care about that affect
the prior_readable_until_ub.  Note that it is only the *down* OSDs that
we have to track here, since everything in the *probe* set we will already
contact during peering (they are still up), guaranteeing that those PGs
are aware of the interval change and are no longer readable in the prior
interval.

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