Give the hook to shut down background executor gracefully

Author: U-FAREAST\ramgupt

src/main/java/com/microsoft/graph/concurrency/DefaultExecutors.java

@@ -144,5 +144,8 @@ public void run() {
     public ILogger getLogger() {
         return logger;
     }
-
+    
+    public void shutDownBackgroundExecutors() {
+    	this.backgroundExecutor.shutdown();
+    }
 }

src/main/java/com/microsoft/graph/concurrency/IExecutors.java

@@ -67,4 +67,9 @@ <Result> void performOnForeground(final int progress,
      */
     <Result> void performOnForeground(final ClientException exception,
                                       final ICallback<Result> callback);
+    
+    /**
+     * Shuts down the background executors
+     */
+    void shutDownBackgroundExecutors();
 }

src/main/java/com/microsoft/graph/core/BaseClient.java

@@ -173,4 +173,8 @@ protected void setHttpProvider(final IHttpProvider httpProvider) {
     public void setSerializer(final ISerializer serializer) {
         this.serializer = serializer;
     }
+    
+    public void shutDown() {
+    	this.executors.shutDownBackgroundExecutors();
+    }
 }

src/main/java/com/microsoft/graph/core/IBaseClient.java

@@ -86,4 +86,9 @@ public interface IBaseClient {
      * Validates this client
      */
     void validate();
+    
+    /*
+     * Shuts down the executors.
+     */
+    void shutDown();
 }