For a RefCounted subclass T, boost::intrusive_ptr<T> must be instantiated as
boost::intrusive_ptr<T>(raw ptr, false) to avoid immortal instances.
Forgetting that final bool parameter is both easy and extremely hard to spot
with desk checking or code review. IntrusivePtr<T> provides constructors that
Do The Right Thing, so we can typedef a subclass T's ptr_t to IntrusivePtr<T>
rather than directly to boost::intrusive_ptr<T>.