Make AsyncEvent behavior less confusing.

[minecrafter]

Events that extend AsyncEvent do not play well with most listeners.

AsyncEvent allows you to callback when you are done with an event. In practice, this concept is almost always confusing and incompatible (i.e. ProxyPingEvent becoming an async event).

There's a couple of solutions to this:

• Deprecate AsyncEvent and add in new Async*Event classes to replace them. These new events would get called before the regular events. This is the easiest approach, and this is what Bukkit does.
• Run all asynchronous handlers before synchronous ones. Tricky to implement as it requires changes to EventBus and requires plugin updates.

[Techcable]

Can we do this without breaking backwards compatibility?
Why not just execute event handlers in parellel, and return once all synchronous handlers are done, but keeping async handlers going?

[minecrafter]

That would essentially produce the same situation that we started with.

We should try to limit backwards-incompatibility as much as possible, so ultimately we may need to make an upstream push for this change.

[Techcable]

How would it break backwards compat?
Instead of having asynchronous events be done when they return and all callbacks are done?
Essentially a fix for buggy plugins.

[Techcable]

We could add a separate event executor, and just callback when complete?