set default timeout to 30 s for async client as well

[Twey]

This brings the behaviour of the async client in line with the sync client.

[seanmonstar]

I think this could break some people, if they typically have responses that stream longer than 30 seconds.

One reason the blocking client has a default and the async one doesn't is that it's much harder to add timeout blocking operations in general, whereas for async, you can wrap any future in something like tokio::time::timeout(fut, dur).

[Twey]

I feel like it's a little bit of a footgun to have subtly different behaviour here. If we expect users to use tokio::timeout maybe there shouldn't be a timeout option built in at all?

[seanmonstar]

I think adding a default is a possibility in a breaking change, which we'll have to do to upgrade hyper to 1.0 anyways.

I'll note that another example is curl, which doesn't set a default: https://curl.se/libcurl/c/CURLOPT_TIMEOUT.html