osd/PG: move down peers out from peer_purged

osd/PG: move down peers out from peer_purged

In purge_strays(), we'll aggressively clear stray_set and
add all related peers into peer_purged.

However, if the corrsponding peer is down and becomes
up again, (unconditionally) adding it to peer_purged
will prevent primary from re-purging it.
(See Active::react(const MNotifyRec& notevt))

On consuming a new osdmap, let's move any down peers out from
peer_purged simutaneously. This way we can lower the risk
of leaving any leftover PGs behind.

Related-to: http://tracker.ceph.com/issues/38931
Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>