mon/OSDMonitor: send_to_waiting() in on_active()

The send_latest() helper may put a message in the waiting_for_map list
if we are not readable, but currently send_to_waiting() is only called
from update_from_paxos(), and it is possible that we may be unreadable
but not then get a map update.

Instead, share the map when we are active.  Do the same for check_subs(),
which is also about sharing the *new* map.  Leave
share_map_with_random_osd() and process_failures() which do are not
concerned with whether this is the latest map or not.

This problem surfaced when we changed the timing of refresh relative to
paxos commit, since update_from_paxos() is now not normally called while
readable; see f1ce8d7c955a2443111bf7d9e16b4c563d445712 and
c711203c0d4b924e5951aa808b243bf06e7ad23a.

Fixes: #5643
Signed-off-by: Sage Weil <sage@inktank.com>
mon/OSDMonitor: clean up waiting_for_map messages on shutdown

Do not leak these.

Fixes: #5643
Signed-off-by: Sage Weil <sage@inktank.com>