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