CLC-2600 Rework FeedUpdateWhiteboard so it can work as a multi-instance ...

...message processor instead of singleton

JIRA: https://jira.media.berkeley.edu/jira/browse/CLC-2600

Rationale for this is what I've been seeing on dev. The non-singleton processor (HotPlate) always works. The singleton (FeedUpdateWhiteboard) listens for a while and then starts to fail in a mode where only the singleton-hosting machine successfully gets messages. 

Workaround is to rewrite FeedUpdateWhiteboard a bit so it can be multi-instance. To make this work I had to change the pattern from get-then-modify the whiteboard to a poll-every-model-then-write the whiteboard (no get required). 

If this works on dev I'll issue a PR for QA.