RFR (S) 8246097: Shenandoah: limit parallelism in CLDG root handling

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8246097

See the details in the RFE.

Webrev:
  https://cr.openjdk.java.net/~shade/8246097/webrev.02/

Testing: hotspot_gc_shenandoah, benchmarks.

-- 
Thanks,
-Aleksey