Use job queue to process Flow notifications

Depends on: https://gerrit.wikimedia.org/r/#/c/156594

Deployment step:

1. deploy this patch along with the patch mentioned above

2. enable job queue on mediawiki: https://gerrit.wikimedia.org/r/#/c/155169/

Change-Id: I72163d5a9e9d8a1eb5612128888ccf51a95846f3
Merge "Use job queue to process Flow notifications"