MAINT-4952: Add IntrusivePtr wrapper for boost::intrusive_ptr.

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>.

indra/llcorehttp/_refcounted.h

@@ -32,6 +32,7 @@
 
 #include "fix_macros.h"
 #include <boost/thread.hpp>
+#include <boost/intrusive_ptr.hpp>
 
 #include "llapr.h"
 
@@ -120,6 +121,24 @@ inline void RefCounted::destroySelf()
 	delete this;
 }
 
+/**
+ * boost::intrusive_ptr may be used to manage RefCounted classes.
+ * Unfortunately RefCounted and boost::intrusive_ptr use different conventions
+ * for the initial refcount value. To avoid leaky (immortal) objects, you
+ * should really construct boost::intrusive_ptr<RefCounted*>(rawptr, false).
+ * IntrusivePtr<T> encapsulates that for you.
+ */
+template <typename T>
+struct IntrusivePtr: public boost::intrusive_ptr<T>
+{
+	IntrusivePtr():
+		boost::intrusive_ptr<T>()
+	{}
+	IntrusivePtr(T* p):
+		boost::intrusive_ptr<T>(p, false)
+	{}
+};
+
 inline void intrusive_ptr_add_ref(RefCounted* p)
 {
 	p->addRef();