RFR: 8222182: ZGC: Use SoftMaxHeapSize to guide GC heuristics

[Per Liden]

The ZDriector currently uses max heap size (-Xmx) as input when deciding 
on when to start a GC cycle. We should start using SoftMaxHeapSize 
instead. Of course, this change depends on JDK-8222145 "Add 
-XX:SoftMaxHeapSize flag", which is currently out for review.

Bug: https://bugs.openjdk.java.net/browse/JDK-8222182
Webrev: http://cr.openjdk.java.net/~pliden/8222182/webrev.0

/Per