o Major features (conflux):
    - Implement Proposal 329 (conflux traffic splitting). Conflux splits
      traffic across two circuits to Exits that support the protocol.
      These circuits are pre-built only, which means that if the pre-built
      conflux pool runs out, regular circuits will then be used.

      When using conflux circuit pairs, clients choose the lower-latency
      circuit to send data to the Exit. When the Exit sends data to the
      client, it maximizes throughput, by fully utilizing both circuits in a
      multiplexed fashion. Alternatively, clients can request that the Exit
      optimize for latency when transmitting to them, by setting the torrc
      option 'ConfluxClientUX latency'.

      Onion services are not currently supported, but will be in arti. Many
      other future optimizations will also be possible using this protocol.
      Closes ticket 40593.