Fix QueueEntry recycle problem. (#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.

(cherry picked from commit 901f76c)

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

@@ -348,8 +348,8 @@ public void run() {
             cbThreadPoolQueueSize.dec();
             journalAddEntryStats.registerSuccessfulEvent(MathUtils.elapsedNanos(enqueueTime), TimeUnit.NANOSECONDS);
             cb.writeComplete(0, ledgerId, entryId, null, ctx);
-            recycle();
             callbackTime.add(MathUtils.elapsedNanos(startTime));
+            recycle();
         }
 
         private final Handle<QueueEntry> recyclerHandle;
@@ -366,6 +366,14 @@ 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);
         }
     }