Update to C++20

[offa]

Update to C++20.

Additional changes:

• Remove MSVC uses wrong field type for strings #177 workaround
• Replace date with std::chrono — lacking library support
• Update Readme
• Project topics

[offa]

A C++20 compatible version of Boost is necessary, otherwise compilation fails with error like this:

error: 'exchange' is not a member of 'std'; did you mean 'std::__atomic_impl::exchange'?