Message queue: block for worker stop

[isaacseymour]

When running the mq/stop, deref all the worker-loop futures.

This is helpful if, for example, the worker processing depends on other parts of a component system. At the moment I'm getting errors where the component system shuts down without being aware of the fact that there are still some worker threads running, meaning that the workers try to access closed resources (for example).

[ptaoussanis]

This looks good, merging now manually- thanks!

[isaacseymour]

Yay, thank you! Do you have a plan for releasing a new version? I'd love to delete all my hacky patching 😄