RFR (S) 8244759: Shenandoah: print verbose class unloading counters

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8244759

Webrev:
  https://cr.openjdk.java.net/~shade/8244759/webrev.01/

Zhengyu, do I use the new timing API correctly?

Sample GC log:
  https://cr.openjdk.java.net/~shade/8244759/gc-log-sample.txt

Testing: hotspot_gc_shenandoah (running)

-- 
Thanks,
-Aleksey