[8u] 2020-07-21, Bulk backports to sh/jdk8

[Aleksey Shipilev]

https://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20200721/webrev.01/

This backports a few changes to 8u, mostly test improvements:
 [backport] 8242375: Shenandoah: Remove ShenandoahHeuristic::record_gc_start/end methods
 [backport] 8247860: Shenandoah: add update watermark line in rich assert failure message
 [backport] 8247751: Shenandoah: options tests should run with smaller heaps
 [backport] 8247754: Shenandoah: mxbeans tests can be shorter
 [backport] 8247757: Shenandoah: split heavy tests by heuristics to improve parallelism

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey