Shared thread-safe owner

[Fedr]

• New class SharedThreadSafeOwner similar by interface and implementation to existed UniqueThreadSafeOwner, but allowing shared ownership.
• mutex is replaced on atomic functions of std::shared_ptr (which are already deprecated in C++20, so we define _SILENCE_CXX20_OLD_SHARED_PTR_ATOMIC_SUPPORT_DEPRECATION_WARNING, but the replacement is not available in Clang).
• Use new class for storing caches in Mesh, PointCloud, Polyline
• The main benefit is that if a mesh with cache is copied, then both copies have shared valid cache.

[andrewerf]

Could firstConstructor ever be true in this branch?

[Fedr]

It is very unlikely, indeed. But it can happen, e.g. if update() method just finished its operation in another thread.