Safe form of release_core system call

[hunhoffe]

Is your feature request related to a problem? Please describe.

Right now, the release_core() system call does not remove/clear an executor from that core (if there is one) so it is up to the process to make sure the core isn't executing anything before making the system call.

Describe the solution you'd like

We probably need to use an interrupt mechanism to clear the core to be released.

Describe alternatives you've considered

You could also make the system call fail (before submitting it to the log, for consistency) if there is an executor running on that core. I'm not sure what mechanism would be best for discerning if there is something running on that core.

Additional context

This feature wasn't added to the initial PR for introducing the release_core() system call due to time constraints; either potential solution shouldn't be too hard to implement.