RFR [8]: Bulk backports to sh/jdk8u

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20180430/webrev.01/

This backports recent work to sh/jdk8u:

 [backport] Testbug: VerifyJCStressTest leaks memory
 [backport] Remove ShenandoahMarkCompactBarrierSet
 [backport] Isolate shenandoahVerifier from stray headers
 [backport] Wipe out ShenandoahStoreCheck implementation
 [backport] Fix broken asserts in ShenandoahSharedEnumFlag
 [backport] Some smallish ShHeapRegionSet changes
 [backport] Shenandoah/SPARC barrier stubs
 [backport] Shenandoah/Zero barrier stubs

Testing: x86_64 {fastdebug|release}, adhoc builds on other platforms

Thanks,
-Aleksey