Some tor relays would report lines like these in their extrainfo
documents:
dirreq-write-history 2011-03-14 16:46:44 (900 s)
This was confusing to some people who look at the stats. It would happen
whenever a relay first starts up, or when a relay has dirport disabled.
Change this so that lines without actual bw entries are omitted.
Implements ticket 2497.
