Update snowflake to persist sessions across proxies
There are two new dependencies, kcp-go and smux, which together make up the inner reliability layer. There's a patch to kcp-go to eliminate dependencies of features we don't use.
This is a Tor Browser ticket but I'm putting it in Circumvention/Snowflake to start to see if there's anything else we want to merge at the same time. Maybe legacy/trac#34042 (moved)?