Add Connection.isConnected()Z API.

Idea from kamcio96 in #1693

api/src/main/java/net/md_5/bungee/api/connection/Connection.java

@@ -51,6 +51,14 @@ public interface Connection
      */
     void disconnect(BaseComponent reason);
 
+    /**
+     * Gets whether this connection is currently open, ie: not disconnected, and
+     * able to send / receive data.
+     *
+     * @return current connection status
+     */
+    boolean isConnected();
+
     /**
      * Get the unsafe methods of this class.
      *