prevent race conditions in run_scheduled_jobs queuing jobs multiple times

we were running into issues where multiple beats queue up the
run_scheduled_jobs task at the same time, and concurrency issues with selecting
scheduled jobs causes both tasks to trigger processing of the job.

Use with_for_update, which calls through to the postgres SELECT  ... FOR UPDATE
which locks other SELECT FOR UPDATES (ie other threads running same code) until
the rows are set to pending and the transaction completes - so the second
thread will not find any rows

app/celery/scheduled_tasks.py

@@ -27,17 +27,24 @@ def remove_csv_files():
 @notify_celery.task(name="run-scheduled-jobs")
 @statsd(namespace="tasks")
 def run_scheduled_jobs():
+    from app import db
     try:
         jobs = dao_get_scheduled_jobs()
         for job in jobs:
             job.job_status = JOB_STATUS_PENDING
-            dao_update_job(job)
-            process_job.apply_async([str(job.id)], queue="process-job")
+            from time import sleep
+            sleep(1)
+            print('SCHEDULING' + str(job.id))
+            # dao_update_job(job)
+            # process_job.apply_async([str(job.id)], queue="process-job")
             current_app.logger.info(
                 "Job ID {} added to process job queue".format(job.id)
             )
+        db.session.add_all(jobs)
+        db.session.commit()
     except SQLAlchemyError as e:
         current_app.logger.exception("Failed to run scheduled jobs", e)
+        db.session.rollback()
         raise