A ThreadPoolExecutor that does not maintain the order of IoEvents.
This means more than one event handler methods can be invoked at the same
time with mixed order. For example, let's assume that messageReceived, messageSent,
and sessionClosed events are fired.
All event handler methods can be called simultaneously.
(e.g. messageReceived and messageSent can be invoked at the same time.)
The event order can be mixed up.
(e.g. sessionClosed or messageSent can be invoked before messageReceived