RFR [10], 2018-08-08, Bulk backports to sh/jdk10

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk10-20180808/webrev.01/

This backports latest work to sh/jdk10:

 [backport] Refactor gc+init logging
 [backport] Check and ensure that Shenandoah-enabled compilations succeed
 [backport] Traversal: Pre-evacuate code-roots and disable barriers for constants
 [backport] Purge support for ShenandoahConcurrentEvacCodeRoots and ShenandoahBarriersForConst
 [backport] Traversal: Purge arraycopy 'optimization'
 [backport] Fix TestGCThreadGroups test
 [backport] Explicit GC should actually uncommit the heap

Testing: tier3_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey