OSD: allow project_pg_history to handle a missing map

OSD: allow project_pg_history to handle a missing map

If we get a peering message for an old map we don't have, we
can throwit out: the sending OSD will learn about the newer
maps and update itself accordingly, and we don't have the
information to know if the message is valid. This situation
can only happen if the sender was down for a long enough time
to create a map gap and its PGs have not yet advanced from
their boot-up maps to the current ones, so we can rely on it

Fixes: #6712
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>