Some chunks that need conversion may fail the loadChunks pass, and
end up entering the world gen code to finish processing.
We solved this on the API level before, but this needs to apply to
all chunk loads when gen=false
This adds a new Future based, Consumer<Chunk> based, and ability
to control whether or not to generate to the Async Chunk API.
Until Async Chunks merges, these API's are still synchronous, but
this commit will allow plugins to start using the API's in use
with the Async Chunks beta.
