osd/PeeringState: fix history.same_interval_since of merge target again

osd/PeeringState: fix history.same_interval_since of merge target again

The symptom looks much like we see in
	https://tracker.ceph.com/issues/37654.

The root cause is that both merge source and target could be
fabricated PGs (aka placeholders), hence merge target's
same_interval_since could remain 0 after merge.

Fix by adjusting history.same_interval_since to last_epoch_clean
reported by these PGs were found to be ready for merge.
This peer is going to be ignored/purged by primary anyway later
when peering is done.

Fixes: https://tracker.ceph.com/issues/45991
Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>