[Unit Testing] Fixed bug with improper severity handling.
[Aux] No longer support for `std::uint8_t` as log attribute type.
[Aux] Disable registering keywords with unsupported underlying types.
[API] First look on synchronized logger.
[API] Added synchronized logger decorator.

It owns logger object and makes its calls thread-safe by using stupid mutex implementation. In the future I’ll replace it by smart formatter-sink thread-safety determiner.
[Unit Testing] Fixed mock warnings.