OSD: allow project_pg_history to handle a missing map

If we get a peering message from prior to a map gap, we
can throw it out since the sending osd will come to its
sense when it finds out about the new map.  We must
throw it out since we can't tell whether it's still valid.

Specifically, the sending osd must be in a situation where
it has gotten up to date maps, but the pg in question
hasn't yet been notified and sends a query/etc based
on the pre-map-gap map.

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