Reduce Memory used by Finished AsyncQueue Instances

ArrayDeque instances have no way to shrink their backing array
which can grow large during query execution and is retained by way
of references in QueryTracker after completion. This resets the
AsyncQueue elements field after finishing to reduce heap usage.

presto-hive/src/main/java/com/facebook/presto/hive/util/AsyncQueue.java

@@ -39,7 +39,7 @@ public class AsyncQueue<T>
     private final int targetQueueSize;
 
     @GuardedBy("this")
-    private final Queue<T> elements;
+    private Queue<T> elements;
     // This future is completed when the queue transitions from full to not. But it will be replaced by a new instance of future immediately.
     @GuardedBy("this")
     private SettableFuture<?> notFullSignal = SettableFuture.create();
@@ -84,6 +84,8 @@ private synchronized void signalIfFinishing()
     {
         if (finishing && borrowerCount == 0) {
             if (elements.size() == 0) {
+                // Reset elements queue after finishing to avoid holding on to the full sized empty array inside
+                elements = new ArrayDeque<>(0);
                 completeAsync(executor, notEmptySignal);
                 notEmptySignal = SettableFuture.create();
             }