Enable concurrency debugging by default, and address issues

[tonyarnold]

:fire: Please don't merge this yet - it's unfinished :fire:

• [x] Build and run tests with concurrency debug active
• [ ] Deprecate confinement contexts in 3.0, with a view to their complete removal in MR 4.0
• [ ] Make Stacks use either private or main queue contexts, or if there is no point without the confinement context, deprecate the stack.
• [ ] Update documentation
• [ ] Add tests for all added methods