[11] 2020-05-28, Bulk backports to sh/jdk11

[Aleksey Shipilev]

https://cr.openjdk.java.net/~shade/shenandoah/backports/jdk11-20200528/webrev.01/

This backports a few easy changes to sh/jdk11:
 [backport] 8245812: Shenandoah: compute root phase parallelism
 [backport] 8245814: Shenandoah: reconsider format specifiers for stats
 [backport] 8245463: Shenandoah: refine ShenandoahPhaseTimings constructor arguments
 [backport] 8245754: Shenandoah: ditch ShenandoahAlwaysPreTouch
 [backport] 8245461: Shenandoah: refine mode name()-s
 [backport] 8245726: Shenandoah: lift/cleanup ShenandoahHeuristics names and properties
 [backport] 8245825: Shenandoah: Remove diagnostic flag ShenandoahConcurrentScanCodeRoots

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey