IntrusiveCounter is a base class used to make a type suitable for use with boost::intrusive_ptr. It has surprising behavior under concurrency, which can lead to bugs such as SERVER-32007. RefCountable is a superior alternative. We should migrate existing uses of IntrusiveCounter to RefCountable, and delete the IntrusiveCounter code. As of this writing, the only remaining subclass of IntrusiveCounter is DocumentSource in the aggregation framework code.