RFR [8], 2018-06-04, Bulk backports to sh/jdk8

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20180604/webrev.01/

Among other things, it makes sh/jdk8 buildable on Mac OS Yosemite.

 [backport] Fixed SA due to code refactoring and merging
 [backport] Rearrange Shenandoah tests into 3 tiers
 [backport] Fix TestFullGCALot test failure
 [backport] Resettable iterators to avoid dealing with copying/assignment compilation differences

Testing: hotspot_tier3_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey