RFR (S) 8242089: Shenandoah: per-worker stats should be summed up, not averaged

[Aleksey Shipilev]

RFE:
  https://cr.openjdk.java.net/~shade/8242089/webrev.01/

Rationale is in the RFE.

Webrev:
  https://cr.openjdk.java.net/~shade/8242089/webrev.01/

Logs before/after:
  https://cr.openjdk.java.net/~shade/8242089/before.txt
  https://cr.openjdk.java.net/~shade/8242089/after.txt

Testing: hotspot_gc_shenandoah, eyeballing logs

-- 
Thanks,
-Aleksey