osd/OSDMap: more improvements to upmap

osd/OSDMap: more improvements to upmap

- add ability of appending a 2nd, 3rd, etc... pair to existing upmaps
  when possible, rather than just continuing to the next PG
- handle the underfull case: we can rm-pg-upmap-items if there exist
  any upmaps which remapped a PG out from an underfull OSD

See-also: http://tracker.ceph.com/issues/37940
Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>
(cherry picked from commit a7d2adf4283cc11ed567b0ad07c4076a50d7d2a0)

Conflicts:
	- various option changes since luminous
	- s/assert/ceph_assert/
        - s/conf./conf->/