Remove the need for gettimeofday_cached(); use monotonic time in ewma code

approx_time() should be done the way we currently do gettimeofday_cached(). gettimeofday_cached() should be removed entirely.

This will improve our performance a little, and improve our code a lot.

edited: this ticket is now only about gettimeofday_cached(); approx_time() will take a different ticket.