Session saving should only print the first N lines of JSON being sent

[marcoct]

tr.send_session_data() should not print everything, but should limits its output to e.g. 10 lines.

[gregory-marton]

Consider making it a result? ipy displays long results in a scroll box.

[riastradh-probcomp]

This is what goes to the session orchestrator's 'info' logger.

It is not immediately clear to me whether this info logging is important. The caller can always get at the session dumps if interested; there are log levels fit for higher verbosity too.

[gregory-marton]

Not needed for any release soon because we're primarily using recipes-based session tracking instead.