quic: rethink connected / disconnected

[marten-seemann]

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We should rethink how we treat QUIC connections. Our current connection model assumes a clear separation between the two states "Connected" and "Disconnected".

QUIC has two interesting properties that dilute this distinction:

1. It's really cheap to keep a QUIC connection alive. The memory overhead is quite small (and has gotten even smaller with recent quic-go releases). We can also reduce the keep-alive frequency to further bring down the computational cost of a QUIC connection.
2. QUIC allows 0-RTT. For the application it doesn't matter that much if we still have a live QUIC connection or if we can just 0-RTT a new connection (at least for replay-safe requests).