Change G1 Full GC heap and thread sizing ergonomics

The heap sizing after the new Full GC differs a bit from the old one, since we now use many threads we will almost always use more than one region after the Full GC is done. This will in turn lead to a more restrictive shrinking of the heap, leaving more regions around after a System.gc() than before.

This can improved by not using as many threads when the heap is small, and one way to scale the number of threads is to use the G1HeapWastePercent value to make sure we don't end up with too many "wasted" regions which in turn will affect the total heap size.