RFR [8u]: Bulk backports to sh/jdk8u

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20180503/webrev.01/

This backports recent performance-critical bugfixes to sh/jdk8u:

 [backport] Shenandoah region/set iterators should not allow copying
 [backport] Set ShenandoahMinFreeThreshold default to 10%

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey