Add changes file for conflux.

changes/ticket40593

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+  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.