osd/PG: discard msgs from down peers

osd/PG: discard msgs from down peers

if a repop is replied after a replica goes down in a new osdmap, and
before the pg advances to this new osdmap, the repop replies before this
repop can be discarded by that replica OSD, because the primary resets the
connection to it when handling the new osdmap marking it down, and also
resets the messenger sesssion when the replica reconnects. to avoid the
out-of-order replies, the messages from that replica should be discarded.

Fixes: http://tracker.ceph.com/issues/19605
Signed-off-by: Kefu Chai <kchai@redhat.com>