osd: do not use incomplete peer for best info/log

osd: do not use incomplete peer for best info/log

For one, their stats are incomplete; if we use them we'll screw up everyone
else.  For another, it doesn't do us any good if they are a bit ahead of
the peers: we/they may not even have the objects their newer log says were
updated.  The only real use is if their log extends farther back in time,
but that is a problem in general that we'll eventually solve in other ways.

On the other hand, having the pg_stats sum only through last_backfill may
not have been the best choice; we could avoid that part of things by adding
a objects_backfilled field.  But this is probably a good idea anyway.

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