add threaded writing

[HerringtonDarkholme]

As suggested in #17

Threaded writing will prevent bad client side blocking.

[palantirtech]

Thanks for your interest in palantir/python-language-server, @HerringtonDarkholme! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[guyzmo]

Hi! Just a thought on this: wouldn't it be better to avoid using threading for that task but rather rely on simpler monothreaded concurrent programming? (using some sort of sugar around select/epoll/kqueue like what asyncio does).

All in all, I just started using the tool and it's ♥

[gatesn]

I agree that threaded writing is not the right solution here, given the rest of the server is currently single-threaded it shouldn't be necessary.

I believe the right approach here might be based around a concurrent results queue that each request writes back to with another thread consuming and sending out the results.