[Bug][CDC] Fix concurrent modify of splits (#3937)

Author: hailin0

seatunnel-connectors-v2/connector-cdc/connector-cdc-base/src/main/java/org/apache/seatunnel/connectors/cdc/base/source/enumerator/IncrementalSourceEnumerator.java

@@ -67,13 +67,13 @@ public void open() {
     }
 
     @Override
-    public void run() throws Exception {
+    public synchronized void run() throws Exception {
         this.running = true;
         assignSplits();
     }
 
     @Override
-    public void handleSplitRequest(int subtaskId) {
+    public synchronized void handleSplitRequest(int subtaskId) {
         if (!context.registeredReaders().contains(subtaskId)) {
             // reader failed between sending the request and now. skip this request.
             return;
@@ -128,7 +128,7 @@ public PendingSplitsState snapshotState(long checkpointId) {
     }
 
     @Override
-    public void notifyCheckpointComplete(long checkpointId) {
+    public synchronized void notifyCheckpointComplete(long checkpointId) {
         splitAssigner.notifyCheckpointComplete(checkpointId);
         // incremental split may be available after checkpoint complete
         assignSplits();