Support "absolute time" for measurements and revert.

After discussion, we decided to revert back to using CFAbsoluteTime for
timing operations. There are pros and cons of each approach, and it
boiled down to basically "keep doing what we were doing since it wasn't
obviously broken". This will keep the metadata timing values consistent.

The things being measured -- latency, connected time -- are really
wall-time measurements, and having them affected by sleep time is odd.