The emper header LockedUnboundedQueue.hpp could depend on different libraries
according to the implementation.

To link those dependencies with everything including LockedUnboundedQueue.hpp
we propagate all emper_dependencies through emper_dep.

And using emper_dep as a dependency seems anyway better than essentially
writing down emper_dep manually each time.

emper_dep essentially is:
(link_with:emper, include_directories: emper_all_include)

This introduces AbstractWorkStealingScheduler which holds the common
work-stealing scheduling strategy.

In debug builds, the schedule() method is now guarded by an assert(),
as due to this change, the schedule() method will no longer work from
everywhere.

This also improves the worker sleep method.

And fix the #include issues reported by the newer IWYU version.

It's 'ns' stupid.