Implement Conflux in C-Tor (Proposal 329)

This is the parent ticket to cover implementation of Conflux (https://gitlab.torproject.org/tpo/core/torspec/-/blob/main/proposals/329-traffic-splitting.txt)

Here's the task breakdown:

• Negotiation (Nick and David)
  • Protover Advertisement for Relays (24h) (#40721 (closed))
  • Link cell parsing + handling (8h) (#40723 (closed))
  • Exit circuit linking (only pre-emptive circuits) (20h)
• Path Usage
  • Ensure paths do not share guards+middles (1h)
  • Maintain pre-linked circuit pool (16h)
• Sequencing and Reordering (Mike and David)
  • Relay header alterations (24h)
  • Reorder queues (likely at edge conn layer) (24h)
  • OOM handling (4h)
• Scheduling Algorithms (Mike)
  • LowRTT (24h)
  • BLEST (40h)
• Resumption (Mike)
  • Sequence tracking and shutdown (no buffering) (24h)
• Shadow Evaluation (Mike)
  • Determine optimum scheduling alg (80h)