WIP: Use bigrquerystorage for downloads

[hadley]

• [x] Update tests so they continue to use json API
• [x] A few basic tests that data is returned correctly — https://github.com/meztez/bigrquerystorage/issues/53
• [x] Make api an argument to collect()
• [x] Provide more efficient db_query_fields() method (using dryRun in bq_perform_query())
• [x] Think through consequences of new billing argument
• [x] Figure out dbFetch()

Closes https://github.com/meztez/bigrquerystorage/issues/46.