[11u] RFR (S) 8252660: Shenandoah: support manageable SoftMaxHeapSize option

[Aleksey Shipilev]

Original RFE:
   https://bugs.openjdk.java.net/browse/JDK-8252660
   https://hg.openjdk.java.net/jdk/jdk/rev/6ab9279c0e99

The patch does not apply cleanly to 11u, because there are conflicts in APIs.

I had to resolve these:
   *) There is no Atomic::store(&size_t, size_t), replaced with OrderAccess::release_store_fence, as 
in other places in 11u.
   *) There is no global SoftMaxHeapSize, replaced it with ShenandoahSoftMaxHeapSize and added 
relevant verifications to make new Shenandoah tests pass.

11u webrev:
   https://cr.openjdk.java.net/~shade/8252660/webrev.11u.01/

Testing: hotspot_gc_shenandoah, tier{1,2} with Shenandoah

-- 
Thanks,
-Aleksey