[backend][local_scheduler] Close database connections before starting new process

This will avoid some concurrency problem especially with SQLite.

Related to the use of the multiprocessing module.

See:
https://code.djangoproject.com/ticket/20562

beat/web/backend/local_scheduler.py

@@ -27,6 +27,7 @@
 
 
 from django.conf import settings
+from django import db
 
 import multiprocessing
 import psutil
@@ -94,6 +95,8 @@ def start_scheduler(settings_module='beat.web.settings.settings', interval=5,
     if getattr(settings, 'LOCAL_SCHEDULER_VERBOSITY', None) is not None:
         args.append(settings.LOCAL_SCHEDULER_VERBOSITY)
 
+    db.connections.close_all()
+
     process = SchedulerProcess(args)
     process.start()
     process.queue.get()