Add simple Async Marker logic to ndb_persistence engine
Add simple Async Marker logic to ndb_persistence engine

Add an entity for use when marking an Async complete and update the
store_async_marker function to create a complete marker entity.

mattsanders-wf/furious,rosshendrickson-wf/furious,andreleblanc-wf/furious,rosshendrickson-wf/furious,Workiva/furious,mattsanders-wf/furious,beaulyddon-wf/furious,andreleblanc-wf/furious,beaulyddon-wf/furious,Workiva/furious