workflows: aborting and skipping

• Adds support in the workflow engine for aborting execution and skipping objects using new functions eng.skipToken() and eng.abortProcessing() that will not save the workflow object. Useful in complex workflow where for example the workflow object is deleted as a final step in a task before aborting/skipping.

• Removes legacy DEBUG flag from engine as logging is anyway logged with the debug loglevel.

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