- Avoid memmoving 0 bytes which might lead to compiler warnings.

- Don't require relays to be entry node AND bridge at the same to time to
  record clients.

- Fix a memory leak when writing dirreq-stats.

- Don't say in the stats files that measurement intervals are twice as long
  as they really are.

- Reduce minimum observation time for requests to 12 hours, or we might
  never record usage.

- Clear exit stats correctly after writing them, or we accumulate old stats
  over time.

- Reset interval start for buffer stats, too.