crimson/osd: reply if pg is absent when handling MQuery

crimson/osd: reply if pg is absent when handling MQuery

in previous commit, the support of `MOSDPGQuery` was dropped. but
if we want to handle `MOSDPGQuery2` properly, we should take
`on_pg_absent()` into consideration when handling `MOSDPGQuery2`.

in this change, we handle `MOSDPGQuery2` by checking the boost event
against `MQuery`, so we don't need to create a new class derived from
`RemotePeeringEvent` for specializing the behavior.

Signed-off-by: Kefu Chai <kchai@redhat.com>