Implement Last Value Cache pattern for iopub

[JohanMabille]

With the current implementation, a client that connects to a kernel that idles never gets the "idle" message published on IOPub. This can be problematic for clients waiting for such a message before sending any request.

A reliable way to fix this is to implement the Last Value Cache pattern: replace the PUB socket with an XPUB socket, which can detect new subscriptions. We can than implement a cache mechanism that sends again the last message upon new subscription.

We must pay attention to set the XPUB_VERBOSE option to true to be able to detect all new subscriptions, not only those to new topics.

[JohanMabille]

See jupyter/jupyter_client#593 for more detail on this.

[JohanMabille]

The PUB socket has been replaced with an XPUB socket in xeus-zmq.