Don't delay uploading a new desc if bw estimate was 0

Don't delay uploading a new desc if bw estimate was 0

When a tor relay starts up and has no historical information about its
bandwidth capability, it uploads a descriptor with a bw estimate of 0.
It then starts its bw selftest, but has to wait 20 minutes to upload the
next descriptor due to the MAX_BANDWIDTH_CHANGE_FREQ delay. This change
should mean that on average, relays start seeing meaningful traffic a
little quicker, since they will have a higher chance to appear in the
consensus with a nonzero bw.

Patch by Roger, changes file and comment by Sebastian.