Towards non-locking model: make 'with-items' work w/o locks

* Fixed 'with-items' to work in non-locking mode. 'concurrency'
  task property has been temporarily dropped since its current
  implementation does not fit into transactional model. It
  needs to be re-implemented based on atomic DB reads/writes
  of a number of currently running actions which is now impossible
  to do with Json field 'runtime_context', most likely a new DB
  field is needed for this.

TODO:
* Fix 'concurrency'
* Fix reverse workflows to work in non-locking mode
* Remove locks

Partially implements: blueprint mistral-non-locking-tx-model
Change-Id: I74bb252533ba4742eb3c7bde73e62ed61ed244bd

mistral/engine/workflow_handler.py

@@ -28,7 +28,7 @@ from mistral.workflow import states
 LOG = logging.getLogger(__name__)
 
 
-_ON_TASK_COMPLETE_PATH = 'mistral.engine.workflow_handler.on_task_complete'
+_ON_TASK_COMPLETE_PATH = 'mistral.engine.workflow_handler._on_task_complete'
 
 
 @profiler.trace('workflow-handler-start-workflow')
@@ -75,7 +75,7 @@ def cancel_workflow(wf_ex, msg=None):
 
 
 @profiler.trace('workflow-handler-on-task-complete')
-def on_task_complete(task_ex_id):
+def _on_task_complete(task_ex_id):
     # Note: This method can only be called via scheduler.
     with db_api.transaction():
         task_ex = db_api.get_task_execution(task_ex_id)