[Standards] QUIC multi-stream

Hi all, I've submitted a PR against XEP-0467 for QUIC multi-stream support. The PR points to a PoC implementation for C2S on both client and server - I should note these both currently only work on Linux. They do run, though, and I'm using them as my "daily driver". You should definitely not do this, the code is demonstrably unsafe (due to implementation, not the spec). This will all need substantial tweaking before it's really where I'd like it, but the key features are that one conversation need not HoL block another. This builds on QUIC's migration etc. The downside of this approach is that it utterly kills XEP-0198. XEP-0198 is predicated on stream state being able to be captured in just two integers and an opaque id. QUIC essentially handles the resumption case, but QUIC connection state is much more complex than two integers, and we lose the common usage of XEP-0198 <r/> - <a/> to drive "single-tick" state. We'll need something new to replace it. If you are familiar with QUIC then you are likely much more familiar with QUIC than I am; please please do review and discuss! Thanks, Dave.