Checker timeout

[Andrew Jeffery]

Executing a check run can take a lot of time for exhaustive checkers, but for the simulation checker it can be unbounded.

While the user can wrap calls with a timeout mechanism, I think it would be a better experience to add a mechanism for timing out the execution.

This could be done at the join level, timing out there, but perhaps better would be within each checker thread, letting the rest of the API be untouched, or perhaps on the job market level, so that there is a single signal for all threads to observe.