parsl.utils.get_all_checkpoints() doesn't find all checkpoints

[drewoldag]

The implementation of get_all_checkpoints() will only look in run_logs/<today>/<run_number>/checkpoint for checkpoint files.

If we didn't define a special run_log directory structure, this would probably be fine, but we did.

Two solutions:

• step back from the date-based directory structure for run logs
• write our own code to ls the directories to look for checkpoint files.