[Debug.hpp] wrap actual logging in log-level constexpr
According to godbolt.org
do { if constexpr(false) { break; } int foo = 42; foo++; } while(false);
does result in code generation for gcc 10.2 and clang 11.0.1 as opposed to
do { if constexpr(false) { int foo = 42; foo++; }} while(false);
which does not result in code generation for both gcc and clang.
And this simple change did indeed significantly increases our echo
benchmark performance. We were probably creating a lot of std::stringstream
objects without ever using them.