RFR: 8222145: Add -XX:SoftMaxHeapSize flag

[Per Liden]

Introduce a manageable SoftMaxHeapSize flag. When set, the GC should 
strive to not grow heap size beyond the specified size. But as the flag 
name suggests, this is a soft limit. In other words, the GC is allowed 
to grow the heap size beyond SoftMaxHeapSize if needed.

There are a few different use cases where this flag could be useful. For 
example, when you want to keep the heap footprint down, while retaining 
the capability to deal with a temporary increase in heap space 
requirement. Or when you want to play it safe, with lots of margin, to 
increase confidence that you will not run into an allocation stall 
because of an unforeseen increase in allocation rate.

The SoftMaxHeapSize should not be allowed to be set to a value smaller 
than min heap size (-Xms) or greater than max heap size (-Xmx). When not 
set on the command-line, this flag should default to the max heap size.

Bug: https://bugs.openjdk.java.net/browse/JDK-8222145
CSR: https://bugs.openjdk.java.net/browse/JDK-8222181
Webrev: http://cr.openjdk.java.net/~pliden/8222145/webrev.1

/Per