workflows: cleaning of engine calls

• Removes generation of another object snapshot when continuing a workflow causing many duplications in the database.

• Reuses the same workflow when continuing a workflow, as it was originally intended.

• Improves current workflow integration to be cleaner, using defined interfaces instead of updating closed variables.

• Adds new restart point, 'first', to re-start a workflow engine from first object and/or first task (callback).

• Removes resetting of engine extra_data when restarting workflow.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>