Build the index serially, rather than in parallel
Build the index serially, rather than in parallel

There's questionable performance benefits to running one worker per file in
parallel, since we should be limited by leveldb write speed either way. In
benchmarks, doing it serially seems to be slightly slower, but thrash the disk
way less. Disk thrashing can cause the http server latency to increase, so I
think it's worth it.

delkyd/sequins,stripe/sequins,stripe/sequins