A ChunkSizeTracker object must be instantiated for each collection.

A private function should also be added to call the ChunkSizeTracker with the relevant information. Later, the ChunkSplitter call can also be added to this function, when we're ready to activate autosplit on the shards. CollectionShardingState already has onUpdateOp, onInsertOp, etc. functions, into which this new private function can be plugged.