Fix QueueEntry recycle problem. (apache#3747)

Descriptions of the changes in this PR:
In the QueueEntry recycle, it only recycles itself to the object pool, but didn't reset some properties.
Like entry, cb, etc. We should reset the filed before recycles itself.

bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/Journal.java

@@ -347,8 +347,8 @@ public void run() {
             cbThreadPoolQueueSize.dec();
             journalAddEntryStats.registerSuccessfulEvent(MathUtils.elapsedNanos(enqueueTime), TimeUnit.NANOSECONDS);
             cb.writeComplete(0, ledgerId, entryId, null, ctx);
-            recycle();
             callbackTime.addLatency(MathUtils.elapsedNanos(startTime), TimeUnit.NANOSECONDS);
+            recycle();
         }
 
         private final Handle<QueueEntry> recyclerHandle;
@@ -365,6 +365,14 @@ protected QueueEntry newObject(Recycler.Handle<QueueEntry> handle) {
         };
 
         private void recycle() {
+            this.entry = null;
+            this.cb = null;
+            this.ctx = null;
+            this.journalAddEntryStats = null;
+            this.journalCbQueuedLatency = null;
+            this.journalCbQueueSize = null;
+            this.cbThreadPoolQueueSize = null;
+            this.callbackTime = null;
             recyclerHandle.recycle(this);
         }
     }