MINOR: time: add conversions to/from nanosecond timestamps

MINOR: time: add conversions to/from nanosecond timestamps

In order to ease the transition away from the timeval used in internal
timestamps, let's first create a few functions and macro to return a
counter from a timeval and conversely, as well as ease the conversions
to/from ns/us/ms/sec to save the user from having to count zeroes and
to think about appending ULL in conversions.