Make accept_hdr_async accept async callback

[jsimonrichard]

This is a feature request:

It should be possible to pass an async callback to the accept_hdr_async function. For example, in a header handling function that I'm writing, I need to check the contents of a cookie against a redis cache on my server, and I'm using an asynchronous library to talk to redis. Being able to pass and async function directly to accept_hdr_async would be really helpful.

Let me know what you think.

Right now I'm going to try to use tokio to circumvent this issue, but please let me know if you have any better ideas. Also, let me know if you need anything from me; if I have the time I might submit a PR.

[sdroege]

See also #159 and sdroege/async-tungstenite#70.

It will require changing the handshake code inside tungstenite to allow running asynchronously.

[jsimonrichard]

Thanks for getting back to me; I looked through the source code for both tokio-tungstenite and tungstenite, and I see what you mean.

[daniel-abramov]

That would be a neat feature to have, but so far no one from us, unfortunately, had time to address it. We actually have a tracking issue for that: #159

I'm going to close this issue since it's a duplicate so that we have one issue to track the progress and state (if any).