[11u] 2020-07-20, Bulk backports to sh/jdk11

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk11-20200720/webrev.01/

This backports a few improvements, notably in Pacer:
 [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}, tier{1,2} with Shenandoah (running, expecting no
new failures)

-- 
Thanks,
-Aleksey