In a couple of places, to implement the OOM-circuit-killer defense
against sniper attacks, we have counters to remember the age of
cells or data chunks.  These timers were based on wall clock time,
which can move backwards, thus giving roll-over results for our age
calculation.  This commit creates a low-budget monotonic time, based
on ratcheting gettimeofday(), so that even in the event of a time
rollback, we don't do anything _really_ stupid.

A future version of Tor should update this function to do something
even less stupid here, like employ clock_gettime() or its kin.