[AIRFLOW-2951] Update dag_run table end_date when state change (#3798)

The existing airflow only change dag_run table end_date value when
a user teminate a dag in web UI. The end_date will not be updated
if airflow detected a dag finished and updated its state.

This commit add end_date update in DagRun's set_state function to
make up tho problem mentioned above.

Author: YingboWang

airflow/api/common/experimental/mark_tasks.py

@@ -208,6 +208,7 @@ def _set_dag_run_state(dag_id, execution_date, state, session=None):
     dr.state = state
     if state == State.RUNNING:
         dr.start_date = timezone.utcnow()
+        dr.end_date = None
     else:
         dr.end_date = timezone.utcnow()
     session.commit()