Re-initialise Netty worker group on plugin restart

This allows the plugin to actually recover from exceptions after a
restart. It also has the side effect of providing nicer error messages
and clearer stack traces to the end user.

Fixes #289

src/main/java/org/logstash/beats/Server.java

@@ -21,9 +21,9 @@ public class Server {
     private final static Logger logger = LogManager.getLogger(Server.class);
 
     private final int port;
-    private final NioEventLoopGroup workGroup;
     private final String host;
     private final int beatsHeandlerThreadCount;
+    private NioEventLoopGroup workGroup;
     private IMessageListener messageListener = new MessageListener();
     private SslSimpleBuilder sslBuilder;
     private BeatsInitializer beatsInitializer;
@@ -35,16 +35,24 @@ public Server(String host, int p, int timeout, int threadCount) {
         port = p;
         clientInactivityTimeoutSeconds = timeout;
         beatsHeandlerThreadCount = threadCount;
-        workGroup = new NioEventLoopGroup();
     }
 
     public void enableSSL(SslSimpleBuilder builder) {
         sslBuilder = builder;
     }
 
     public Server listen() throws InterruptedException {
+        if (workGroup != null) {
+            try {
+                logger.debug("Shutting down existing worker group before starting");
+                workGroup.shutdownGracefully().sync();
+            } catch (Exception e) {
+                logger.error("Could not shut down worker group before starting", e);
+            }
+        }
+        workGroup = new NioEventLoopGroup();
         try {
-            logger.info("Starting server on port: " +  this.port);
+            logger.info("Starting server on port: {}", this.port);
 
             beatsInitializer = new BeatsInitializer(isSslEnable(), messageListener, clientInactivityTimeoutSeconds, beatsHeandlerThreadCount);
 
@@ -63,16 +71,20 @@ public Server listen() throws InterruptedException {
         return this;
     }
 
-    public void stop() throws InterruptedException {
+    public void stop() {
         logger.debug("Server shutting down");
         shutdown();
         logger.debug("Server stopped");
     }
 
     private void shutdown(){
         try {
-            workGroup.shutdownGracefully().sync();
-            beatsInitializer.shutdownEventExecutor();
+            if (workGroup != null) {
+                workGroup.shutdownGracefully().sync();
+            }
+            if (beatsInitializer != null) {
+                beatsInitializer.shutdownEventExecutor();
+            }
         } catch (InterruptedException e){
             throw new IllegalStateException(e);
         }