osd: subscribe to the newest osdmap when reconnecting to a monitor

This is mostly relevant in testing clusters, but it ensures that an OSD
disconnecting from the monitor at the wrong time will still see any recent
map updates and prevent accidental loss of map injection into the OSD cluster.
Fixes: #9219

Signed-off-by: Greg Farnum <greg@inktank.com>
Merge pull request #2499 from ceph/wip-9219-giant

wip-9219: subscribe to the newest osdmap when reconnecting to a monitor

Reviewed-by: Sage Weil <sage@redhat.com>