Refactor to remove local catabalancer state

[mjh1]

Now that we're using a DB for the node stats we can remove the internal state for this data. The only state we still maintain in catabalancer is the list of nodes updated by serf.

[mjh1]

Had to make a small change not to use the serf member updates, catalyst-api doesn't receive those, we can rely on the DB data instead and exclude nodes where the data is stale.