Use std::atomic<std::shared_ptr> when applicable

[MHebes]

std::atomic has been specialized for std::shared_ptr in C++20.

Using the std::atomic_* function instead causes a compiler warning at least on MSVC for C++ 20.

Theoretically one could choose which ASP to use based on compiler support. I don't know the right #ifdefs to check off the top of my head but I'm sure they exist!