Use single queue

ondemand/service.py

@@ -77,7 +77,7 @@ device_ref = fbdb.reference('devices').child(config['device_id'])
 calibrations = Calibrations(config['device_id'], client=client)
 
 # settings
-queue_mode = QUEUE_THREAD
+queue_mode = QUEUE_SINGLE
 threshold_mode = THRESHOLD_FIXED
 
 db_path = config.get('localDatabase', os.path.join(device_path, 'files.db'))