asyncio support, or SansIO support

[stalkerg]

Basically, nowadays we are working in async/await frameworks like aiohttp, sanic and etc. For seamless integration, the python-ldap should support async/await interface. I propose to make this one.

SansIO can give some other benefits, like easy support trio or any other transport.

[mistotebe]

This is blocked by the request/response API overhaul. If you're interested in helping get that off the ground, have a look at #464