Improved shutdown for BufferedRateExecutor

 /**
  * Copyright © 2016-2018 The Thingsboard Authors
- *
+ * <p>
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
+ * <p>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p>
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
@@ -26,6 +26,7 @@ import java.util.UUID;
 import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
 import java.util.concurrent.LinkedBlockingDeque;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.TimeUnit;
@@ -45,6 +46,7 @@ public abstract class AbstractBufferedRateExecutor<T extends AsyncTask, F extend
     private final ExecutorService callbackExecutor;
     private final ScheduledExecutorService timeoutExecutor;
     private final int concurrencyLimit;
+    private volatile boolean stopped;
 
     protected final AtomicInteger concurrencyLevel = new AtomicInteger();
     protected final AtomicInteger totalAdded = new AtomicInteger();
@@ -106,7 +108,10 @@ public abstract class AbstractBufferedRateExecutor<T extends AsyncTask, F extend
             AsyncTaskContext<T, V> taskCtx = null;
             try {
                 if (curLvl <= concurrencyLimit) {
-                    taskCtx = queue.take();
+                    taskCtx = queue.poll(1, TimeUnit.SECONDS);
+                    if (taskCtx == null) {
+                        continue;
+                    }
                     final AsyncTaskContext<T, V> finalTaskCtx = taskCtx;
                     logTask("Processing", finalTaskCtx);
                     concurrencyLevel.incrementAndGet();
@@ -167,7 +172,7 @@ public abstract class AbstractBufferedRateExecutor<T extends AsyncTask, F extend
         }
     }
 
-    protected int getQueueSize(){
+    protected int getQueueSize() {
         return queue.size();
     }
 }