As #14829 nears completion we need to have support in Envoy for trying QUIC connections and failing over to TCP. This sort of "try something and fail over" is also needed for mobile and dynamic forward proxy for ipv6 vs ipv4, and for mobile cellular vs wifi so we need something which can at worst handle the full grid of [QUIC vs TCP][IPv4 vs IPv4][cellular vs wifi]

The plan of record is to have a connection pool which seamlessly tries one after another, hiding the complexity from both the TCP proxy code and the router, and just selects the best available connection and uses it.