osd/PeeringState: handle race condition of RemoteReservationRevoked event for Backfilling state
Currently When `RemoteReservationRevoked` races with
`MOSDPGBackfill::OP_BACKFILL_FINISH`, the PG sate gets updated to
`backfill_wait`, backfill is suspended and the event is discarded. Note
that we have not rescheduled a backfill to happen again. This can lead
to the PG getting stuck in `backfill_Wait` forever.