Phase out CollectiveMonitorStateInternal

[srithon]

We currently have CollectiveMonitorState separate from CollectiveMonitorStateInternal, but in order for the code to function we had to make CollectiveMonitorStateInternal publicly accessible. Because CollectiveMonitorState is just a RwLock wrapper around CollectiveMonitorStateInternal, we could probably flatten this hierarchy and get rid of the distinction quite easily.