Fencing is about data visibility in multi-processor system and ordering
loads and stores. When reference counter hits zero, current thread is
last one to hold the reference, and it's actually about to discard the
object. It doesn't have to care about data becoming visible to other
processors. Nor is there reason to assume that speculative loads in
destructor would yield stale data, because object was already assumed
to be "stable" when it was used just before reference counter hit zero.

…squashed].