need support asyncio

[ArtemIsmagilov]

I believe that the library needs asynchronous support - this will significantly improve the performance and efficiency of the library

[tobixen]

I agree, but I probably have no capacity to follow up this year. Design ideas and pull-requests are welcome. I'm in for anyhting that doesn't break backward-compatibility.

[tobixen]

#610

It's been a long journey, experimenting and looking into different ways of doing this, but finally I've settled for a "sans-io"-approach to it.