Shenandoah: refactor reference updating closures and related code

We have a block in ShenandoahHeap::maybe_update_heap_refs that is irrelevant after JDK-8231086. Additionally, "resolve and update" paths are really only used by STW GCs, and thus do not require atomic updates. This leads to considerable simplifications in the code, and improves performance on the common paths (especially in fastdebug builds that drop many irrelevant asserts).