Implementations for ChunkStatuses below FULL are supposed to always
return ProtoChunk instances. However, since we used the last completed
status, it could return LevelChunk.
To resolve this, follow Vanilla behavior of tracking chunk
completions by status and replace old ProtoChunk statuses with
ImposterProtoChunk when the chunk generates to FULL.
Additionally, implement an optimisation for retrieving full chunks
by storing a map of pos -> LevelChunk. This requires only a simple
map lookup to occur for full chunks which are loaded.
