* Remove the server_id from session table, change it to worker_id on session_connection
This is step 1 in updating the worker table. The worker is becoming a public resource so it must have
a public id. We drop the server_id column on the session_connection table and add the worker_id column
so we can use the wt_public_key type. Since db migrations are applied while the controller is offline,
it should be safe to assume all session connections are not in a running state, thus allowing us to null
out all the worker ids in the session_connection table.
For now we don't have very good enforcement of the wt_public_id constraints, since the id is currently
defined in the worker config, but this will soon change and the worker's id will be generated by the
controller.
* Refactor the different repo methods into separate files
* Move DefaultLiveness to the repository.go file to be shared across workers and controllers.
irenarindos
Todd
Jeff Mitchell
Jim
Michael Gaffney