Defer Flow notification triggering till end of request

When creating a new topic, the notification is triggered before the page is created, we need to defer the triggering after the page creation

This will nedd core patch: https://gerrit.wikimedia.org/r/#/c/155151/2

Because event could be pushed to job queue via a job queue, so will need
support for adding deferred upates with a deferred update

Change-Id: Iecca33f6a3836f73f68105f4e7ec29907c0ad910