RFR: Shenandoah Partial GC with generational/LRU policies

[Christine Flood]

http://cr.openjdk.java.net/~chf/GenerationalPartialGC/webrev.01/

I've made the changes Shade requested and passed all of the tests,
including the suggested additions, with the exception
of gc/shenandoah/ShenandoahJNICritical.java which is also broken in a clean
build.

Christine