* Remove Node from data model; standardize on Worker
This commit does a lot of things, but in general it attempts to simplify
the data model by getting rid of the Node and NodeRole models. Instead,
these will use the Worker model, which itself has individual boolean
fields for role types.
Get rid of roleType in some FreeWorker methods
rename NodeService to WorkerRegistry
simplify the freeworker interface
fix the tests
* Remove DeleteWorker method from workerJobService
Switch Serverless from using BoltDB to Postgres as metadata store.
Previously, the controller stored all metadata to BoltDB. This implements SQLDB (currently Postgres flavored) as the backing store for metadata. This will allow us to have multiple instances of the controller running for HA, and to easily inspect and repair the contents of the metadata store.
Unfortunately, it was not straightforward to keep the BoltDB implementation working alongside the SQL one, so it will be removed in a later patch. Once that's done, the SQL implementation should allow for a number of simplifications of the schemar and balancer interfaces.
Database migration is built directly into the application by embedding the migration files and logic from the `soda` command line tool. When connecting to the RDBMS, the app will always attempt to create the necessary database and apply any outstanding migrations.
Integration tests truncate all tables upon start, but *not* at the end, so the state of the database can be inspected after integration tests.
Had to refactor some of the controller's background tasks to make sure they get properly shut down on controller exit.
