Multiplex - one client splits traffic across multiple proxies

This is the dual of legacy/trac#25601 (moved).

It would be a great feature if a client could be connected to multiple snowflake proxies at once, and split traffic across all of them. Like the Conflux proposal for Tor. It would be a hedge against getting assigned a single slow proxy.

This would require interposing some kind of sequencing and reliability layer, with possible retransmissions.

A potential benefit for privacy is that an individual snowflake proxy only sees a subset of a client's traffic flow.

However, instantly making N WebRTC connections is a tell for traffic classification.