fix writeLac memory leak

Author: wenbingshen

bookkeeper-server/src/main/java/org/apache/bookkeeper/client/PendingWriteLacOp.java

@@ -19,6 +19,8 @@
 
 import java.util.BitSet;
 import java.util.List;
+
+import io.netty.util.ReferenceCountUtil;
 import org.apache.bookkeeper.client.AsyncCallback.AddLacCallback;
 import org.apache.bookkeeper.net.BookieId;
 import org.apache.bookkeeper.proto.BookkeeperInternalCallbacks.WriteLacCallback;
@@ -112,6 +114,8 @@ public void writeLacComplete(int rc, long ledgerId, BookieId addr, Object ctx) {
 
         if (receivedResponseSet.isEmpty()){
             completed = true;
+            ReferenceCountUtil.release(toSend);
+            toSend = null;
             cb.addLacComplete(lastSeenError, lh, ctx);
         }
     }