add http datastore api

This adds a db backend implementation that targets a RESTy API server.

The intent here is to eventually eliminate the mysql specific implementation and only have heap (for quick dev) and httpdb for real life. To ease the transition I've kept db/mysql.js so we can test the httpdb implementation on a subset of instances in parallel with the current setup. I already moved db_patcher and the schema patches to the db server codebase because those are run manually in production. The httpdb.js implementation is working but could probably use some tidying up.

The db api server currently resides https://github.com/dannycoates/fxa-auth-db-server

Remaining todos are:

- [ ] how to deploy to dev
- [ ] how to deploy to stage / prod
- [ ] move fxa-auth-db-server to a mozilla org
- [ ] eliminate extraneous mysql bits
- [ ] document the DB REST API

Regarding deployment I'm particularly interested in what @gene1wood and @mostlygeek think. I expect we'll have other internal services like this for other parts of the system. Previously, Gene and I discussed a strategy where the internal services are behind an internal ELB with DNS for routing.

@rfk @chilts @ckarlof for feedback on the code / approach.