[apache#1462] fix(server): Memory may leak when flushQueue is full (a…

…pache#1463)

### What changes were proposed in this pull request?

Release the memory when a flush event is dropped.

### Why are the changes needed?

For [apache#1462](apache#1462)

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Existing UTs.

server/src/main/java/org/apache/uniffle/server/DefaultFlushEventHandler.java

@@ -69,6 +69,9 @@ public DefaultFlushEventHandler(
   public void handle(ShuffleDataFlushEvent event) {
     if (!flushQueue.offer(event)) {
       LOG.error("Flush queue is full, discard event: " + event);
+      // We need to release the memory when discarding the event
+      event.doCleanup();
+      ShuffleServerMetrics.counterTotalDroppedEventNum.inc();
     } else {
       ShuffleServerMetrics.gaugeEventQueueSize.inc();
     }