feat: emit with timeout

This feature allows to send a packet and expect an acknowledgement from
the server within the given delay.

Syntax:

```java
socket.emit("hello", "world", new AckWithTimeout(5000) {
    @Override
    public void onTimeout() {
        // ...
    }

    @Override
    public void onSuccess(Object... args) {
        // ...
    }
});
```

Related:

- https://github.com/socketio/socket.io-client-java/issues/309
- https://github.com/socketio/socket.io-client-java/pull/517

src/main/java/io/socket/client/AckWithTimeout.java

@@ -0,0 +1,35 @@
+package io.socket.client;
+
+import java.util.Timer;
+import java.util.TimerTask;
+
+public abstract class AckWithTimeout implements Ack {
+    private final long timeout;
+    private final Timer timer = new Timer();
+
+    /**
+     *
+     * @param timeout delay in milliseconds
+     */
+    public AckWithTimeout(long timeout) {
+        this.timeout = timeout;
+    }
+
+    @Override
+    public final void call(Object... args) {
+        this.timer.cancel();
+        this.onSuccess(args);
+    }
+
+    public final void schedule(TimerTask task) {
+        this.timer.schedule(task, this.timeout);
+    }
+
+    public final void cancelTimer() {
+        this.timer.cancel();
+    }
+
+    public abstract void onSuccess(Object... args);
+    public abstract void onTimeout();
+
+}

src/main/java/io/socket/client/Socket.java

@@ -210,8 +210,32 @@ public class Socket extends Emitter {
                 Packet<JSONArray> packet = new Packet<>(Parser.EVENT, jsonArgs);
 
                 if (ack != null) {
-                    logger.fine(String.format("emitting packet with ack id %d", ids));
-                    Socket.this.acks.put(ids, ack);
+                    final int ackId = Socket.this.ids;
+
+                    logger.fine(String.format("emitting packet with ack id %d", ackId));
+
+                    if (ack instanceof AckWithTimeout) {
+                        final AckWithTimeout ackWithTimeout = (AckWithTimeout) ack;
+                        ackWithTimeout.schedule(new TimerTask() {
+                            @Override
+                            public void run() {
+                                // remove the ack from the map (to prevent an actual acknowledgement)
+                                acks.remove(ackId);
+
+                                // remove the packet from the buffer (if applicable)
+                                Iterator<Packet<JSONArray>> iterator = sendBuffer.iterator();
+                                while (iterator.hasNext()) {
+                                    if (iterator.next().id == ackId) {
+                                        iterator.remove();
+                                    }
+                                }
+
+                                ackWithTimeout.onTimeout();
+                            }
+                        });
+                    }
+
+                    Socket.this.acks.put(ackId, ack);
                     packet.id = ids++;
                 }
 
@@ -405,6 +429,12 @@ public class Socket extends Emitter {
             this.subs = null;
         }
 
+        for (Ack ack : acks.values()) {
+            if (ack instanceof AckWithTimeout) {
+                ((AckWithTimeout) ack).cancelTimer();
+            }
+        }
+
         this.io.destroy();
     }