Add shutdown flag.

proxy/src/main/java/com/velocitypowered/proxy/VelocityServer.java

@@ -49,6 +49,7 @@ public class VelocityServer implements ProxyServer {
     private KeyPair serverKeyPair;
     private final ServerMap servers = new ServerMap();
     private final CommandManager commandManager = new CommandManager();
+    private boolean shutdown = false;
 
     private final Map<UUID, ConnectedPlayer> connectionsByUuid = new ConcurrentHashMap<>();
     private final Map<String, ConnectedPlayer> connectionsByName = new ConcurrentHashMap<>();
@@ -134,7 +135,16 @@ public class VelocityServer implements ProxyServer {
         return this.cm.createWorker();
     }
 
+    public boolean isShutdown() {
+        return shutdown;
+    }
+
     public void shutdown() {
+        Preconditions.checkState(!shutdown, "Server already shut down");
+        shutdown = true;
+
+        logger.info("Shutting down the proxy...");
+
         this.cm.shutdown();
     }