When sbws has measured less than 60% of relays, write a header with progress
In legacy/trac#28062, 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
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