Make default async future completion executor a true multithread executor

Author: zoewangg

.changes/next-release/bugfix-AWSSDKforJavav2-963df91.json

@@ -0,0 +1,5 @@
+{
+    "category": "AWS SDK for Java v2", 
+    "type": "bugfix", 
+    "description": "Make default `asyncFutureCompletionExeuctor` a true multi-threads executor. See [#968](https://github.com/aws/aws-sdk-java-v2/issues/968)"
+}

core/sdk-core/src/main/java/software/amazon/awssdk/core/client/builder/SdkDefaultClientBuilder.java

@@ -267,11 +267,17 @@ protected AttributeMap childHttpConfig() {
      * Finalize which async executor service will be used for the created client.
      */
     private Executor resolveAsyncFutureCompletionExecutor(SdkClientConfiguration config) {
-        Supplier<Executor> defaultExecutor = () ->
-                new ThreadPoolExecutor(0, 50,
-                                       10, TimeUnit.SECONDS,
-                                       new LinkedBlockingQueue<>(10_000),
-                                       new ThreadFactoryBuilder().threadNamePrefix("sdk-async-response").build());
+        Supplier<Executor> defaultExecutor = () -> {
+            ThreadPoolExecutor executor = new ThreadPoolExecutor(50, 50,
+                                                                 10, TimeUnit.SECONDS,
+                                                                 new LinkedBlockingQueue<>(10_000),
+                                                                 new ThreadFactoryBuilder()
+                                                                     .threadNamePrefix("sdk-async-response").build());
+
+            // Allow idle core threads to time out
+            executor.allowCoreThreadTimeOut(true);
+            return executor;
+        };
 
         return Optional.ofNullable(config.option(FUTURE_COMPLETION_EXECUTOR))
                        .orElseGet(defaultExecutor);