RFR: 8249801: Shenandoah: Clear soft-refs on requested GC cycle

[rkennke at redhat.com]

Shenandoah is not clearing soft-refs on a requested (by System.gc() or
implicitely) GC. We should do that. It is breaking
java/util/logging/Logger/bundleLeak/BundleTest.java that exects soft-
refs to evevtually be cleared by System.gc().

Bug:
https://bugs.openjdk.java.net/browse/JDK-8249801

Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8249801/webrev.01/

Testing: hotspot_gc_shenandoah, tier1&2 with Shenandoah

Ok?

Thanks,
Roman