all: Use multiple QUIC streams (fixes #8879)

[calmh]

Work in progress, to be described more fully in time, but in principle:

• support multiple streams on a single connection at the protocol level
• use multiple streams for concurrent requests
• hope for improved greatness

[calmh]

So, there were a few hopes on this one:

1. Primarily, that it would improve performance by multiplexing multiple requests in separate streams. However, I could manufacture no condition under which I saw a performance difference compared to what we already do.
2. That it would serve, API-wise, as a base for the multiple TCP connection work I intended to do. However, the mechanism for how that needs to work is so different from what we can do in QUIC that there's really no overlap.

As such, I don't see a reason for this to move forward. I'm leaving it here with this tombstone should someone see a reason to resurrect it in the future.

As part of the multiple connection work it's also possible to run multiple, separate QUIC connections, should someone see the need to do so.