Sort by recent activity

I tried about 100 replies to topics and the sortby recent activity
failed once.  Any subsequent updates to that topic won't make it appear
on top of the list, it turns out that we are passing the same updated
value in $old & $new to topicListLastUpdatedIndex->onAfterUpdate(),
that results in no cache update.

It looks like TopicListEntry is already having a 'workflow_last_update_timestamp',
TopicListEntry can be looked up by either topic_id or topic_list_id, it could be
that CachingObjectMapper::fromStorageRow() is loading a previous loaded instance
which contains workflow_last_update_timestamp with old value.

This would probably fix the problem: always overwrite 'workflow_last_update_timestamp'

Followup: We should probably always have workflow_last_update_timestamp in
TopicListEntry for consistency

Change-Id: I6bd20f1ccda86083b1ab11d02a1fa0153c136ceb
Merge "Sort by recent activity" into frontend-rewrite