Replace bluelet with asyncio

Split from #621:
> There's a new asyncio library that is very similar in purpose to Bluelet but better-maintained. There is a backport called [trollius](https://pypi.python.org/pypi/trollius).

>While factoring out Bluelet is not a bad idea, I feel obligated to add here that—in the medium term—perhaps Bluelet should no longer exist. I wrote it in the days before [asyncio](https://docs.python.org/3/library/asyncio.html), and in many ways the standard library or [Curio](https://github.com/dabeaz/curio) now does what Bluelet does but better in every way. Replacing Bluelet with a modern off-the-shelf async library could be a fun project if anyone's interested in getting deep into it.
https://github.com/beetbox/beets/pull/1966#issuecomment-485148429