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

`std::atomic` has been [specialized for `std::shared_ptr` in C++20](https://en.cppreference.com/w/cpp/memory/shared_ptr/atomic2).

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 `#ifdef`s to check off the top of my head but I'm sure they exist!