Partial results and "continue" functionality for interrupted scenarios

[davidslater]

Currently, if a scenario is interrupted due to any factor (e.g., OOM or KeyboardInterrupt), no results are written to disk.

This is particularly painful in long-running scenarios that fail several examples into a run, or if a run needs to be stopped for a server reboot.

An alternative would be to stream results to an intermediate file (e.g., write a line before example or batch to file and flush), and then combine the intermediate results to the final results json if the scenario completes successfully.

It would likely require a continue method to do the remaining examples.

[lcadalzo]

MLFlow may handle this for us