Enhancement: Make it possible to resume experiments if the server crashes

[some-rando-rl]

A requirement for this will be to annotate checkpoints after resuming in such a way that indicates that they weren't produced in a single run.

Perhaps this annotation could be accomplished by incrementing a "run id" number, and maybe an optional (or required?) field that captures whether the previous run terminated cleanly or not. If we do include that additional field, it will need to be regarded as best-effort however, as there's no such thing as a tamperproof mechanism for determining whether or not there was a clean exit between runs.

[AechPro]

TODO:
save reward normalization stats with other stuff
change experiment tools to enable resuming from checkpoint

[some-rando-rl]

This will depend on completion of #40