WIP async loading

[mezz]

This adds two config flags that are disabled by default:

1. enables async loading of JEI, which means it will not block the world load and it will continue loading in the background for a while before it's available to use.
2. enables parallel plugin loading. this is more of an experimental config option that will break stuff. Plugins can opt into this more intelligently by implementing the new IAsyncModPlugin instead of IModPlugin.