RFR (S) 8251336: Shenandoah: assert "only get here when SATB active" after JDK-8244997

[Kim Barrett]

> On Aug 11, 2020, at 10:41 AM, Coleen Phillimore <coleen.phillimore at oracle.com> wrote:
> 
> 
> Kim's suggestion for this change looks really good.  I'm re-testing this now:
> 
> open webrev at http://cr.openjdk.java.net/~coleenp/2020/8251336.02/webrev
> bug link https://bugs.openjdk.java.net/browse/JDK-8251336

In release_oop_handles, because Service_lock is a "special" lock and
touched in lots of places, I'd prefer the deletion not happen under
the lock.  (I realize this uglifies the code a little bit.)

Other than that, this looks good.