A list of listeners for implementing a concurrency friendly observable object.

Listeners are registered once via addListener and then may be invoked by enqueueing and then dispatching events.

The API of this class is designed to make it easy to achieve the following properties

• Multiple events for the same listener are never dispatched concurrently.
• Events for the different listeners are dispatched concurrently.
• All events for a given listener dispatch on the provided executor.
• It is easy for the user to ensure that listeners are never invoked while holding locks.

The last point is subtle. Often the observable object will be managing its own internal state using a lock, however it is dangerous to dispatch listeners while holding a lock because they might run on the directExecutor() or be otherwise re-entrant (call back into your object). So it is important to not call dispatch while holding any locks. This is why enqueue and dispatch are 2 different methods. It is expected that the decision to run a particular event is made during the state change, but the decision to actually invoke the listeners can be delayed slightly so that locks can be dropped. Also, because dispatch is expected to be called concurrently, it is idempotent.