[PATCH] Use std::chrono::high_resolution_clock for the Timer class instead of getrusage().

getrusage() is a very expensive system call and involves several
kernel locks. This prevents multithreaded applications from scaling
effectively. Using std:chrono::high_resolution_clock is portable
and very low cost in most cases.

(cherry picked from commit fa43f4bf7974affb6b7fc7b23b5ebde108cce5b0)
Signed-off-by: Markus Blatt <markus@dr-blatt.de>