I merged the *_thread file into mod.rs, since mod.rs is pretty empty. This should be a better layout for when more threads are introduced.

Can you elaborate why?

mod.rs was only going to be used for mod declarations and re-exporting idb_thread::IndexedDBThreadFactory and internal engine stuff wanted access to idb_thread so I figured it's probably cleaner to remove the file entirely. Since indexeddb and webstorage aren't exposed publicly this doesn't really change much on that front so I figured this would reduce some of the path complexity.