ADT: Support copying of IntrusiveRefCntPtr objects

This was partially supported but untested for RefCountedBase (the
implicit copy assignment would've been problematic - so delete that) and
unsupported (would not have compiled, because std::atomic is
non-copyable) for ThreadSafeRefCountedBase (implement similar support
to RefCountedBase)

Fix the test that had a copy ctor for the derived object but called
RefCountBase's default ctor from that copy ctor - which meant it wasn't
actually testing RefCountBase's copy semantics.