Backport `initial_max_send_streams` support to 0.14.x

[Ralith]

This backports the relevant changes from https://github.com/hyperium/h2/issues/731. Happy to drop the changed default if minimizing disruption to existing users is preferred, but I suspect the current behavior is detrimental to most users (see e.g. https://github.com/facebook/buck2/issues/316) and the tweaked default will make things better.

[seanmonstar]

Just as a general note, v1 came out 1.5 years ago, and I'm reducing the amount of maintenance done to 0.14. I know there's less initial work if someone else backports, but if a new patch disrupts someone unexpectedly, I need to put more maintenance in a dying branch.

That's not to say we can't do this...

[Ralith]

Yeah. Luckily it's a clean backport, so risk should be pretty low. Just need a decision on the changed default.

[seanmonstar]

I'd rather not roll out a change to defaults in 0.14.x. Support from active contributors has ended.

[Ralith]

It's been long enough that the downstream I care about has since updated to 1.x anyway.