Implement the C++20 Synchronization Library

[hkaiser]

C++20 will have a set of synchronization primitives (especially counting_semaphore, latch, and barrier) that we should implement (see https://wg21.link/P1135, https://wg21.link/N4849).

The main issue here is that we already have objects with the same name but with possibly diverging interfaces. We should investigate a possible migration from our current API to something compatible with C++20.

• hpx::barrier (Adding C++20 barrier #4636)
• hpx::counting_semaphore, hpx::binary_semaphore (Adding C++20 counting semaphore API #4634)
• hpx::latch (Adding C++20 latch API #4635)
• hpx::condition_variable, hpx::condition_variable_any, hpx::cv_status (Adding C++20 counting semaphore API #4634)

[hkaiser]

This can be closed now.