[logging] reduce logging overhead

[tijyojwad]

Suggestions from @ahehn-nv -

1.

You could avoid the construction of the std::string by directly operating on the stream:

std::ostream& operator << (std::ostream& os, LogLevel level)
{
    switch(level)
    {
       case critical: os << "CRITICAL"; break;
       ...
    }
    return os;
}
or, if you dislike overloading operators, a similar function void add_prefix(std::ostream& os, LogLevel level).

2.

We could actually base the whole logger on overloading << instead of macros and std::strings, providing a syntax like:

 log(LogLevel::debug) << "here's some debug integer: " << some_integer << std::endl;
Advantages:

common C++ syntax.
messages become almost no-ops if a higher logging level is selected.
Disadvantage:

more templates -> Slightly higher compilation times.