RFR (S) 8243494: Shenandoah: set counters once per cycle

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8243494

There are several places where we reuse the counters for different purposes, for example
full_gc_roots. There are places where we accidentally count things twice, for example conc_mark. It
would be hard-checked by JDK-8239786, so we need to fix it before then.

Fix:
  https://cr.openjdk.java.net/~shade/8243494/webrev.01/

Testing: hotspot_gc_shenandoah, eyeballing logs

-- 
Thanks,
-Aleksey