This PR enables the use of placeholders in an upstream's Dial address.
A Dial address must represent precisely one socket after replacements.
See also #998 and #1639.
* Begin WIP integration of HTTP/3 support
* http3: Set actual Handler, make fakeClosePacketConn type for UDP sockets
Also use latest quic-go for ALPN fix
* Manually keep track of and close HTTP/3 listeners
* Update quic-go after working through some http3 bugs
* Fix go mod
* Make http3 optional for now