Fix data race in ds (pingcap#1161)

close pingcap#1160

Author: hongyunyan

utils/dynstream/parallel_dynamic_stream.go

@@ -83,6 +83,15 @@ func (s *parallelDynamicStream[A, P, T, D, H]) Start() {
 }
 
 func (s *parallelDynamicStream[A, P, T, D, H]) Close() {
+	// clean pathMap, to avoid sending into a closed channel after close()
+	{
+		s.pathMap.Lock()
+		for k := range s.pathMap.m {
+			delete(s.pathMap.m, k)
+		}
+		s.pathMap.Unlock()
+	}
+
 	for _, ds := range s.streams {
 		ds.close()
 	}