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

[Florian Weimer]

* 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.

I think G1 supports SoftMaxHeapSize as well, but it does not run
back-to-back collections once the live set exceeds the configured size.
Obviously, it's likely that those back-to-back collections only burn CPU
cycles without bringing the heap size down below SoftMaxHeapSize.

ZGC seems to be have more like Shenandoah with this patch, I think.
There's no G1-style backoff, either.

Thanks,
Florian
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