LevelProvider: Nuked the complicated mess out of level providers

Level providers are now cut back to just an interface to a world's data. They don't keep their own chunk registries or any stupid shit like that because the Level already does that.

This furthers the goal of being able to move level I/O off the main thread, and also drastically decreases the complication of implementing level providers.

src/pocketmine/level/format/io/leveldb/LevelDB.php

@@ -527,7 +527,6 @@ class LevelDB extends BaseLevelProvider{
 	}
 
 	public function close(){
-		$this->unloadChunks();
 		$this->db->close();
 	}
 }