RFR (XS) 8241985: Shenandoah: simplify collectable garbage logging

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8241985

Webrev:
  https://cr.openjdk.java.net/~shade/8241985/webrev.01/

See the log examples in the issue itself.

Testing: hotspot_gc_shenandoah, eyeballing gc logs

-- 
Thanks,
-Aleksey