This avoids TSAN warnings triggered by concurrent writes to the global variable, even though there is no race (as noone reads the variable).
This removes dependencies on Boost.Thread and Boost.Chrono, as well as their dependencies and potentially allows to test more compilers. In particular, this removes the dependency on Boost.Lexical cast, which no longer compiles with gcc 4.6 and 4.7.
This commit adds C++20 atomic_ref implementation, documentation and tests.
