src: modernize likely/unlikely hints

[anonrig]

An attempt to modernize C++ code to use [[likely]] and [[unlikely]] available after C++20.

Reference: https://en.cppreference.com/w/cpp/language/attributes/likely

This pull-request also disables readability/braces rule from cpplint, since it doesn't support C++20. Main branch of cpplint supports it, but unfortunately they didn't release a new version in the last 2 years.