Max concurrent stream improvements

[tomchristie]

Right, there was some stuff I didn't quite get correct in #89

Absurdly I'd forgotten to use http2=True in my testings. This is the script I'm using now for this...

import asyncio
import httpcore


async def main():
    async with httpcore.AsyncConnectionPool(http2=True) as client:
        await asyncio.gather(*[request(client, idx) for idx in range(1000)])

async def request(client, idx):
    http_version, status_code, reason_phrase, headers, stream = await client.request(
        method=b'GET',
        url=(b'https', b'example.org', 443, b'/'),
        headers=[(b'host', b'example.org:443')]
    )

    try:
        body = b''.join([chunk async for chunk in stream])
    finally:
        await stream.aclose()


asyncio.run(main())

The changes here are:

• We need to use the local settings to determine how many concurrent streams we can issue, not the remote settings.
• The stream_id = self.h2_state.get_next_available_stream_id() needs to be part of the semaphore-limited wrapped code, or else it stops incrementing once we've hit the concurrency limit. As a result I've moved the wrapping layer out from AsyncHTTP2Stream onto AsyncHTTP2Connection instead.

[yeraydiazdiaz]

Nice! 👍