mon/PGMonitor: improve debugging on PGMap updates slightly

Chasing #7652
Signed-off-by: Sage Weil <sage@inktank.com>
mon/PGMap: send pg create messages to primary, not acting[0]

For erasure pools, these may not match.

In the case of #7652, this caused pg_create messages to be send
indefinitely.  register_pg() added it to the list for acting_primary, and
when we got the (non-creating) pg stat update we removed it from the list
for acting[0].

Fixes: #7652
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1404 from ceph/wip-7652

mon: fix infininte pg create msgs for erasure pools

Reviewed-by: Samuel Just <sam.just@inktank.com>