[8u] 2020-08-11, Bulk backports to sh/jdk8

[Aleksey Shipilev]

https://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20200811/webrev.01/

This backports a bunch of fixes/improvements to sh/jdk8:

 [backport] 8248041: Shenandoah: pre-Full GC root updates may miss some roots
 [backport] 8249560: Shenandoah: Fix racy GC request handling
 [backport] 8249801: Shenandoah: Clear soft-refs on requested GC cycle
 [backport] 8241574: Shenandoah: remove ShenandoahAssertToSpaceClosure
 [backport] 8241007: Shenandoah: remove ShenandoahCriticalControlThreadPriority support

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey