Fix a couple of smaller issues with gathering statistics.
- 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.