Report bandwidth usage aggregated over a longer period

NUM_SECS_BW_SUM_INTERVAL == 15 minutes seems pretty short; can we go up to 4 hours or 6 hours or so?

Note that while 0.2.2.x was still on the network we couldn't do this:

/** How large are the intervals for which we track and report bandwidth use? */
/* XXXX Watch out! Before Tor 0.2.2.21-alpha, using any other value here would
 * generate an unparseable state file. */
#define NUM_SECS_BW_SUM_INTERVAL (15*60)

But now, nobody should be downgrading to 0.2.2.x; this change should be approximately trivial.