add http datastore api

The intent here is to eventually eliminate the mysql specific
implementation and only have heap (for quick testing) 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.