[11] RFR 2020-06-16: Critical backports to sh/jdk11

[Aleksey Shipilev]

https://cr.openjdk.java.net/~shade/shenandoah/backports/jdk11-20200616/webrev.01/

This is a bunch of critical backports to stabilizing sh/jdk11:
  [backport] 8247310: Shenandoah: pacer should not affect interrupt status
  [backport] 8247474: Shenandoah: Windows build warning after JDK-8247310
  [backport] 8247358: Shenandoah: reconsider free budget slice for marking
  [backport] 8247560: Shenandoah: heap iteration holds root locks all the time

Testing: hotspot_gc_shenandoah; tier{1,2,3} with Shenandoah enabled

-- 
Thanks,
-Aleksey