[RFC] parallel plugin fetching

Summary:
Use Python 3.4's `asyncio` to do plugin fetching in parallel. Right
now we don't try to parallelize any build or cache operations; those are still
done in series to keep things simple. We also don't do anything fancy with the
console; fetches will still print as they go, though reups have been modified a
little to make sure their outputs don't interleave.

Before we push something like this, we'll add to `third-party` to make sure
that at least 3.3 is supported. The same `yield from` coroutines won't work in
3.2, so we may end up dropping support for that.

Test Plan: All tests pass.

Reviewers: sean

Differential Revision: https://phabricator.buildinspace.com/D58