Robust erroring for processes or executions that modify the same resource

[krs85]

Just wanted to make a central issue for this because it just keeps popping up in other issues 😬

Let's be sure we panic! or unreachable! or something whenever:

• Processes within the same execution tree (sharing the same root process) modify the same resource
• Processes in disparate execution trees (other executions in the cache) modify the same resource