Disable heap iteration for Shenandoah in SA

[Roman Kennke]

As Per pointed out in reviews, we cannot safely iterate objects in SA,
because current implementation is size-based. This might just crash.
Let's do the same as ZGC for now and disable it, and revert the other
pieces.

http://cr.openjdk.java.net/~rkennke/disable-sa-heapdump/webrev.00/

Testing: hotspot_gc_shenandoah, hotspot_serviceability

Ok?