[11] 2020-03-05, Bulk backports to sh/jdk11

[Roman Kennke]

Looks good!

Thank you!
Roman

> https://cr.openjdk.java.net/~shade/shenandoah/backports/jdk11-20200305/webrev.01/
> 
> This is hopefully the last batch of backports for April release:
>  [backport] 8240511: Shenandoah: parallel safepoint workers count should be ParallelGCThreads
>  [backport] 8240534: Shenandoah: ditch debug safepoint timeout adjustment
> 
> Testing: hotspot_gc_shenandoah {fastdebug,release}
>