Right now, Tor routers don't save the maxima values from the
bw_history_t between sessions.  That's no good, since we use those
values to determine bandwidth.  This code adds a new BWHist.*Maximum
set of values to the state file.  If they're not present, we estimate
them by taking the observed total bandwidth and dividing it by the
period length, which provides a lower bound.

This should fix bug 1863.  I'm calling it a feature.