fs.readdir takes too damn long

[clee]

For reference, the initial issue that I found discussing the fs.readdir problem (fs.readdir takes way too long on large directories, where "large" means "more than 10k files" at least in my testing):

nodejs/node-v0.x-archive#388

This looks like the current nodejs issue tracking support for a streaming/iterative readdir:
nodejs/node#583

And it seems like it's currently blocked on:
libuv/libuv#416

Once libuv/libuv#416 and nodejs/node#583 are both merged, we should probably move to using the streaming readdir instead for better performance.

[clee]

I should probably write some tests to confirm the performance improvement, but the entire codebase is now using the Promise-based API and should perform MUCH better now. Closed with 22decfb.