storage: support recovering from backend

We want the KV to support recovering from backend to avoid
additional pointer swap. Or we have to do coordination between
etcdserver and API layer, since API layer might have access to
kv pointer and use a closed kv.

etcdserver/server_test.go

@@ -867,10 +867,10 @@ func TestConcurrentApplyAndSnapshotV3(t *testing.T) {
 
 	be, tmpPath := backend.NewDefaultTmpBackend()
 	defer func() {
-		be.Close()
 		os.RemoveAll(tmpPath)
 	}()
 	s.kv = dstorage.New(be, &s.consistIndex)
+	s.be = be
 
 	s.start()
 	defer s.Stop()