workflows: revamp session handling

• Changes the way the db.session is called in BibWorkflowEngine and BibWorkflowObject models to use a session_manager decorator.

• Adds new classmethods to BibWorkflowObject to create objects and revisions.

• New BibWorkflowObject's are now by default initiated with version set to INITIAL (from RUNNING).

• RUNNING version is only set when a BibWorkflowObject is actually being executed in a BibWorkflowEngine.

• Removes the workflow status FINISHED, as it was too ambiguous compared to COMPLETED. If a workflow contains data objects that have not reached FINAL version, the workflow status is set to HALTED.

• Cleans some of the log messages in the engine.

• Reworks what happens when a workflow is asked to *fully* restart. It will take the INITIAL version of every associated object and run it through the workflow from the start.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>