When sbws has measured less than 60% of relays, write a header with progress
In legacy/trac#28062 (moved), we remove the link to the bandwidth file when fewer than 60% of relays are measured.
Instead, we could write a bandwidth file containing a header field with:
- number of relays measured / target number of relays / total number of relays
- percentage of relays measured / target percentage
Edit: the bandwidth file would not contain any relays - just the header