Changing a job post-submission

[SteVwonder]

We need to define what we want to allow to be changed post-submission (good starting point: look at what Slurm allows), a method/protocol for actually modifying the jobspec post-submission, and how modules will be notified of and handle these changes.

[stale]

This issue has been automatically marked as stale because it has not had activity for 365 days. It will be closed if no further activity occurs within 14 days. Thank you for your contributions.

[garlick]

closing due to newer discussion in #3416