workflows: clean engine calls

* No longer generates another object snapshot when continuing a
  workflow causing many duplications in the database.

* The same workflow is re-used when continuing a workflow, as 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).

* Cleans up obvious, unneeded comments.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>