server: Save consistency index and term to backend even when they decrease

Reason to store CI and term in backend was to make db fully independent
snapshot, it was never meant to interfere with apply logic. Skip of CI
was introduced for v2->v3 migration where we wanted to prevent it from
decreasing when replaying wal in
https://github.com/etcd-io/etcd/pull/5391. By mistake it was added to
apply flow during refactor in
https://github.com/etcd-io/etcd/pull/12855#commitcomment-70713670.

Consistency index and term should only be negotiated and used by raft to make
decisions. Their values should only driven by raft state machine and
backend should only be responsible for storing them.

etcdutl/snapshot/v3_snapshot.go

@@ -476,6 +476,6 @@ func (s *v3Manager) updateCIndex(commit uint64, term uint64) error {
 	be := backend.NewDefaultBackend(s.outDbPath())
 	defer be.Close()
 
-	cindex.UpdateConsistentIndex(be.BatchTx(), commit, term, false)
+	cindex.UpdateConsistentIndex(be.BatchTx(), commit, term)
 	return nil
 }