[11] 2020-06-04: Bulk backports to sh/jdk11

[Aleksey Shipilev]

https://cr.openjdk.java.net/~shade/shenandoah/backports/jdk11-20200604/webrev.01/

This backports a few safe-ish performance improvements.

Changes:
 [backport] 8246100: Shenandoah: walk roots in more efficient order
 [backport] 8246097: Shenandoah: limit parallelism in CLDG root handling

Testing: hotspot_gc_shenandoah {fastdebug,release}, tier{1,2} with Shenandoah

-- 
Thanks,
-Aleksey