Enforce proper memory ordering in refcnt_rele() and refcnt_finalize()

Enforce proper memory ordering in refcnt_rele() and refcnt_finalize()

Make refcnt_rele() and refcnt_finalize() order memory operations so that
preceding loads and stores happen before 1->0 transition. Also ensure
that loads and stores that depend on the transition really begin only
after the transition has occurred. Otherwise the object destructor might
not see the object's latest state.

OK bluhm@