expose the shutdown method of the event loop group

To exit normally all non-daemon threads of the java process are supposed
to have stopped. TcpClientSession creates an static class member
EventLoopGroup which in turn instantiates worker threads. These need to
be stopped as a part of shutting down the server.

This patch exposes the shutdown of the EventLoopGroup as a static
function intended to be called by some central function as a part of
server shutdwon.

Author: Peter Svensson

src/main/java/com/github/steveice10/packetlib/tcp/TcpClientSession.java

@@ -318,4 +318,15 @@ private static void createTcpEventLoopGroup() {
 
         Runtime.getRuntime().addShutdownHook(new Thread(() -> EVENT_LOOP_GROUP.shutdownGracefully()));
     }
+
+    public static void shutdown() {
+        if (EVENT_LOOP_GROUP != null) {
+            try {
+                EVENT_LOOP_GROUP.shutdownGracefully().sync();
+            } catch (InterruptedException e) {
+               e.printStackTrace();
+            }
+
+        }
+    }
 }