Load all chunks via async chunk system.

Currently we use the async chunk loading system only when players trigger
chunk loading. If a chunk is loaded for any other reason it goes through
a separate codepath. This means if a player has trigged a chunk load and
before the chunk loads something else wants the same chunk we will load it
twice and throw away the second result. With this change we instead use
the sync processing feature of the AsynchronousExecutor to load the chunk
which will pull it out of the queue if it was already queued to load. This
means we only ever load a chunk once. Note that this does not move chunk
generation off the main thread, we fallback to the vanilla chunk loading
system for chunks that have not yet been generated.