Intended as replacement for the Mutex/ConditionVar based barrier
built into the exiting Lumiera thread handling framework and used
to ensure safe hand-over of a bound functor into the starting new
thread. The standard requires a comparable guarantee for the C++17
concurrency framework, expressed as a "synchronizes_with" assertion
along the lines of the Atomics framework.
While in most cases dedicated synchronisation is thus not required
anymore when swtiching to C++17, some special extended use cases
remain to be addressed, where the complete initialisation of
further support framework must be ensured.
With C++20 this would be easy to achieve with a std::latch, so we
need a simple workaround for the time being. After consideration of
the typical use case, I am aiming at a middle ground in terms of
performance, by using a yield-wait until satisfying the latch condition.
