BwScanner does not clear stream data between slices

The output files ( ./data/scanner.{n}/scan-data/bws-* ) contain entries from previous slices.

The default relays-per-slice is 50; but the number of entries per output file grows by about 50 per slice.

Example: the output file generated for the third slice would contain the entries for the first and second slices as well.

It looks like the sqlite databases are not being cleared between speedrace() runs.