Configure TURN servers for the proxy and/or client

[keroserene]

This may require spinning up our own TURN servers somewhere.

In most cases, the public ip candidates given by STUN servers are sufficient for peers to negotiate a working p2p path. But sometimes, maybe 14% of the time [1], this isn't enough -- usually due to symmetric NAT / lack of port preservation, and a TURN relay is required as a last resort.

Also, once snowflake proxies & clients are multiplexed, perhaps the likelihood that TURN continuous to be absolutely required for every pair of peers greatly decreases and we might be fine again. More thought is needed here.

[arlolra]

This is now https://trac.torproject.org/projects/tor/ticket/25596