Attach term metadata to UpdatePosition command

MongoDB replication protocol differs from Raft in that voters of the new primary doesn't reject fetched oplog entries from the old primary. Those stale entries will be rolled back eventually. However, if that voter reports its progress of the stale entries to the old primary before the rollback, even very rarely, the stale entries may be considered as committed. This ticket is to forward voter's current term to the old primary, making it step down.