use fast replication

Using [pouchdb-load](https://github.com/nolanlawson/pouchdb-load) and some static files hosted on S3 and dumped via [pouchdb-dump-cli](https://github.com/nolanlawson/pouchdb-dump-cli), I was able to get performance comparable to what we had with the janky "fast replication" pre-3.0.3.

I'll also write a README update explaining how this works.