onack(packet) {
    const ack = this.acks[packet.id];
    if (typeof ack !== "function") {
      return;
    }
    delete this.acks[packet.id];
    if (ack.withError) {
      packet.data.unshift(null);
    }
    ack.apply(this, packet.data);
  }
Called upon a server acknowledgement.

@param packet
@private