RFR [8], 2018-08-08, Bulk backports to sh/jdk8

[Roman Kennke]

Yep, looks good! Go! Thanks, Roman

> http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20180808/webrev.01/
> 
> This backports recent work to sh/jdk8:
> 
>  [backport] Refactor gc+init logging
>  [backport] Check and ensure that Shenandoah-enabled compilations succeed
>  [backport] Purge support for ShenandoahConcurrentEvacCodeRoots and ShenandoahBarriersForConst
>  [backport] Fix TestGCThreadGroups test
>  [backport] Explicit GC should actually uncommit the heap
> 
> Testing: hotspot_tier3_gc_shenandoah {fastdebug|release}
> 
> Thanks,
> -Aleksey
>