[8u] 2020-07-24, Bulk backports to sh/jdk8

[Aleksey Shipilev]

https://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20200724/webrev.01/

This backports a bunch of improvements to sh/jdk8:
 [backport] 8249953: Shenandoah: gc/shenandoah/mxbeans tests should account for corner cases
 [backport] 8248652: Shenandoah: SATB buffer handling may assume no forwarded objects
 [backport] 8247367: Shenandoah: pacer should wait on lock instead of exponential backoff
 [backport] 8247593: Shenandoah: should not block pacing reporters
 [backport] 8249649: Shenandoah: provide per-cycle pacing stats

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey