Investigate async web frameworks

[aecay]

We need both an http server and an http client for mite. The client is needed for running the tests themselves; a server is needed for the prometheus exporter. If we do fancy things with javascript/client side testing, websockets, etc., we may eventually want a http/ws server in mite for that as well.

Desiderata:

• support a wide variety of protocols (ssl, websockets, ...)
• easy to maintain (3rd party > python only > C)
• easy to extend (grokkable code, responsive upstream if applicable)
• fast
• similar implementation for server and client (ideally same library, at least same broad approach)

There are various existing implementations:

acurl

Pros	Cons
the status quo	requires C compilation
fast	does not support websocket/etc without extension
gives us the timing info we need	client only (as it stands)

aiohttp

Pros	Cons
wide ecosystem of plugins (websockets etc)	requires C compilation (but distribution question is solved)
client and server	doesn't provide timing info as it stands

sanic

Pros	Cons
apparently very fast	server only (as it stands)
pure python(!)	server only (again because it's a big deal)

There's a lot to consider, and I haven't formed any clear views yet. My current favorite idea though is to look to sanic for inspiration for writing a client library in pure python (and rely on sanic for serving). Sanic demonstrates that good performance is possible without C. A pure python implementation allows us to add whatever bells and whistles we want to (around timing etc.), while being easy to maintain. Ideally, we can duplicate the functionality of acurl with less code (and zero C). A second choice would be to use acurl as the basis for a server library.

[jb098]

I think I agree, I'd quite like to lean hard into the Async stuff, as much as I love Flask. Let's try and replace our Flask integrations with Sanic and see how we get on.