Implement JEP 65: ensure IOPub messages are not missed by using XPUB

[jmcphers]

Kallichore should implement JEP 65, which provides a way for clients to ensure IOPub messages are not missed due to a race condition.

JEP 65 is documented here: https://jupyter.org/enhancement-proposals/65-jupyter-xpub/jupyter-xpub.html

The corresponding work on the Ark side has already been merged: posit-dev/ark#577

[jmcphers]

There's an open question here of how to figure out if the kernel supports JEP 65 (i.e. whether it is ever going to send the iopub_welcome message). @DavisVaughan pointed out that this itself is the subject of another JEP, 92. https://github.com/jupyter/enhancement-proposals/blob/master/92-jupyter-optional-features/jupyter-optional-features.md

[lionel-]

Ark now implements the feature flag: posit-dev/ark#584 (comment).

Our kernel-info-reply contains a supported_features list that includes "iopub_welcome" to indicate support for the welcome message defined by JEP 65.

[DavisVaughan]

See also posit-dev/ark#673 about subscribing before connecting particularly posit-dev/ark#673 (comment). All this would come into play if kallichore implements JEP 65