a better approach for triggering world generation without using leveldb.ErrNotFound

[schphe]

currently dragonfly's internal (*World).loadChunk function appears to be written with leveldb in mind

hence those who want to implement their own third-party world providers need to import leveldb package and return leveldb.ErrNotFound in order to trigger world generation for the chunk

to keep things more neat, i think we should introduce a better approach for triggering it or a chunk.ErrNotFound error