feat(http2): add initial_max_send_streams method to HTTP/2 client builder

[magurotuna]

This adds initial_max_send_streams method to the HTTP/2 client builder.

The value set via this method will then be forwarded to h2::client::Builder::initial_max_send_streams. If not set, the default value defined in h2 crate will be used in case some users might rely on this behavior. However, in the following version we plan to explicitly set 100 as a reasonable default value (see hyperium/h2#731 for rationale on why this can be considered a reasonable value).

[seanmonstar]

I had this thought, and I'd love to hear other opinions. I can be convinced either way. But:

Should the feature be exposed in 1.2, and the default changed in 1.3? I wonder if there's value in giving people some time to specify their preferred option, before perhaps limiting them more than expected.

At the same time, getting the default to people faster may be more helpful to more people.

[magurotuna]

Given that hyper is forming the foundation of HTTP in the Rust ecosystem, being conservative makes more sense to me. Your first idea, exposing the builder method in the next minor version and then setting the default value in one more next minor version, sounds a better one. I'd be happy to make a change to this PR to remove the default value part.

From our (developers of Deno) perspective, our eventual goal is to fix the fetch implementation that's powered by reqwest create. So it would be ideal if we could ship hyper's next version including this new builder method, and incorporate the new version into reqwest finally. (Then we can configure the initial_max_send_streams value on our side)

[seanmonstar]

Alright, yea, sounds good to me too. If you want to remove the smaller default here, we can include this in 1.2 with a note that the default will change.

[magurotuna]

OK, so now the part of setting the default value has been removed with a note stating the default value may change.