crimson/osd: fix the order in OSD::update_heartbeat_peers()

crimson/osd: fix the order in OSD::update_heartbeat_peers()

before this chance, we do following things in background and in
parallel:

* add peers from pg map
* and peers from whole osdmap
* remove dead peers

but we should prioritize the peer candidates, apparently, the ones from
pgmap should have higher priority. as they are more likely to be used in
cluser and they are more important to current OSD, as they serve the
same set of PGs.

so we need to do these things in a proper order:

1. add peers from pg map
2. and peers from whole osdmap
3. remove dead peers

Signed-off-by: Kefu Chai <kchai@redhat.com>