CLC-2600 Simplify FeedUpdateWhiteboard write logic and correct it so it ...

...won't be inconsistent

Per comment from Ray, the state of the whiteboard could be inconsistent with the old code (one merged model's feed-changed message could be processed while another one was still updating, and the second one's feed-changed message would not result in a new whiteboard save). In this revision, we just expire the whiteboard when a feed-changed message comes through, then rebuild the whiteboard whenever the UI requests it, caching that result for the default 35 minutes.