Implement Comm for asyncio

[mrocklin]

In some benchmarks that involve many small messages I've found that we spend a non-trivial amount of time in Tornado's IOStream code (see stack overflow question). I've become a bit curious about uvloop. One way to explore this would be to implement a Comm and Backend (see docs) for asyncio and then see how it performs both alone, and after installing uvloop as the default event loop.

cc @pitrou who may have some high-level guidance about whether or not this is a good idea and how best to implement this with asyncio, and @jcrist who might enjoy doing this work.

[mrocklin]

I'm playing around with this today, but don't expect to get particularly far.

[mrocklin]

Today I got as far as this branch: master...mrocklin:asyncio-comm

This look ok, but unsurprisingly don't function correctly.

[jakirkham]

Maybe it's worth opening the PR anyways so it is easier to discuss (even if it doesn't totally work yet).

[jcrist]

Closing, duplicate of #4513.