This PR changes the the assign-capacity queue to prioritize streams that
are send-ready.

This is necessary to prevent a lockout when streams aren't able to proceed while
waiting for connection capacity, but there is none.

Closes hyperium/hyper#3338

Co-authored-by: dswij <[email protected]>

This change causes GOAWAYs to be issued to misbehaving connections which for one reason or another cause us to emit lots of error resets.

Error resets are not generally expected from valid implementations anyways.

The threshold after which we issue GOAWAYs is tunable, and will default to 1024.

This speeds up loading blocks in cases where we have many headers already.

Calculate the amount of allowed CONTINUATION frames based on other
settings.

    max_header_list_size / max_frame_size

That is about how many CONTINUATION frames would be needed to send
headers up to the max allowed size. We then multiply by that by a small
amount, to allow for implementations that don't perfectly pack into the
minimum frames *needed*.

In practice, *much* more than that would be a very inefficient peer, or
a peer trying to waste resources.

See https://seanmonstar.com/blog/hyper-http2-continuation-flood/ for
more info.