workflows: do not take snapshot on continue

* A new object snapshot is no longer taken continuing a
  workflow as the initial snapshot taken is enough to reconstruct
  the workflow.

* start_workflow() now has an additional parameter to pass
  the current object to the next workflow. Additionally,
  the id_parent is retained when creating a copy.

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