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

[Aleksey Shipilev]

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}

-- 
Thanks,
-Aleksey