Async command map building

[perseus5]

Builds the command map async, focuses on fixing the root problem described in #2938 instead of just ignoring it. Leaf said he had a better idea but since I had this finished already.. thought might as well PR it.

Needs testing from someone who suffers from lag spikes when players connect/change worlds.
Shows up as CommandDispatcher.a() in sampler reports.

[aikar]

Impl looks perfectly fine to me. is it running fine for you behaviorally? Tested with a plugin using the event?

[perseus5]

Had someone test it, looks like it broke tab complete completely. no errors in console either. https://i.gyazo.com/1c9616a53411e3acd73159de77a159ef.png

Guess it's not possible?

[electronicboy]

I did actually wanna look into this a while back, but there was a thing or two which I'd figured I'd have to look into, but it's doable

[perseus5]

As Billy pointed out, I was using submit instead of execute for the async task. Issue still persisted, but I also noticed that I was using postToMainThread for the sync task. That method does not do as the name suggests, so that has also been changed to execute. After testing, it looks like these were the correct changes as it retains the same functionality as before, just now off the server thread.