RFR (S) 8252660: Shenandoah: support manageable SoftMaxHeapSize option

[Aleksey Shipilev]

RFE:
   https://bugs.openjdk.java.net/browse/JDK-8252660

Since JDK 13, there is a SoftMaxHeapSize option that basically instructs GC heuristics to target 
that particular heap size, while leaving the headroom for allocation spikes and other contingencies. 
It makes sense to support this in Shenandoah.

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

Testing: hotspot_gc_shenandoah; tier{1,2} with Shenandoah; new tests; adhoc experiments with jcmd

-- 
Thanks,
-Aleksey