osd/PG: rename require_same_or_newer_map -> is_same_or_newer_map

This avoids confusion with the OSD method of the same name, and better
matches what the function tests (and does not do).

Signed-off-by: Sage Weil <sage@inktank.com>
osd: mark down connections from old peers

Close out any connection with an old peer.  This avoids a race like:

- peer marked down
- we get map, mark down the con
- they reconnect and try to send us some stuff
- we share our map to tell them they are old and dead, but leave the con
  open
...
- peer marks itself up a few times, eventually reuses the same port
- sends messages on their fresh con
- we discard because of our old con

This could cause a tight reconnect loop, but it is better than wrong
behavior.

Other possible fixes:
 - make addr nonce truly unique (augment pid in nonce)
 - make a smarter 'disposable' msgr state (bleh)

Signed-off-by: Sage Weil <sage@inktank.com>
Merge branch 'wip-osd-map' into next

Fixes: 4369
Backport: bobtail
Reviewed-by: Samuel Just <sam.just@inktank.com>