Implement smarter database migration

[de-code]

The current database migration is quite crude - drop and rebuild tables if the schema version doesn't match. Data is then getting rebuild. As postgresql requires a lock when dropping tables, all existing connections need to be dropped (which is why the service is stopped before every re-deployment).

Alembic can be used to manage database migrations:
http://alembic.zzzcomputing.com/en/latest/

(also see comments in JIRA)