As we close in on #2557 we want to start thinking about upstream support for QUIC.

The main use case may be for mobile (envoyproxy/envoy-mobile#1033) but it could be used in general for forward proxying and cross-data center hops.

v1 would be hard-coded QUIC with no failover, but for use on the internet it would eventually need to include things like QUIC vs TCP racing, 0-rtt credential caching, alt-scv cache, etc.