Avoid use of CFAbsoluteTime as much as possible.

A helper class SPDYStopwatch is introduced here to abstract common
time management duties from the rest of the code.

Since CFAbsoluteTimeGetCurrent() is affected by clock and timezone
changes, it's use should be avoided. mach_absolute_time() is unaffected
by clock changes, but is susceptible to overflow issues (which can be
worked around) and doesn't increment while the system is asleep (which
cannot be worked around). However, it has more appropriate behavior.

Our use of CFAbsoluteTimeGetCurrent() for the timers is unavoidable,
since Cocoa timers are based on it. This seems like it would present
undesirable behavior for our deferrable and connect timers if the clock
were to change, but there's not much to do about it.
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.
Merge pull request #87 from kgoodier/review/absolute-time

Avoid use of CFAbsoluteTime as much as possible.