CLC-2903 Rate-limiting feed_changed messages

https://jira.media.berkeley.edu/jira/browse/CLC-2903

Only send feed_changed messages (which expire the FeedUpdateWhiteboard) once every 10s. In case FeedUpdateWhiteboard gets out of sync and misses a feed_changed because of this limiting, we also expire FeedUpdateWhiteboard on a 5 minute delay.