TODO for JEP65:

Subscribe to all subscription messages

// Set xpub_verbose option to 1 to pass all subscription messages (not only unique ones).
m_publisher.set(zmq::sockopt::xpub_verbose, 1);

From the guide (): https://zguide.zeromq.org/docs/chapter5/

by default, the XPUB socket does not report duplicate subscriptions, which is what you want when you’re naively connecting an XPUB to an XSUB. Our example sneakily gets around this by using random topics so the chance of it not working is one in a million. In a real LVC proxy, you’ll want to use the ZMQ_XPUB_VERBOSE option that we implement in Chapter 6 - The ZeroMQ Community as an exercise.

IOPub needs to listen on its socket

How do we listen to both XPUB and channel messages?

Have the forwarding thread own the socket? Need to send messages via that thread too...