RFR (XS) 8247358: Shenandoah: reconsider free budget slice for marking

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8247358

See the details in JIRA.

Fix:
  https://cr.openjdk.java.net/~shade/8247358/webrev.01/

Testing: hotspot_gc_shenandoah, benchmarks

-- 
Thanks,
-Aleksey