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

[Aleksey Shipilev]

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