Cherry-pick: Concurrent class unloading related changes

[Zhengyu Gu]

I would like to cherry-pick following concurrent class unloading related 
changes to shenandoah/jdk:

JDK-8227755: Need more than 2 distinct CodeCache unloading cycles
JDK-8227866: Shenandoah: Split weak root processing and class unloading 
in parallel cleaning task
JDK-8227635: Shenandoah: SHBSA::load_at() needs to deal IN_NATIVE load
JDK-8228532: Shenandoah: Implement SBSA::try_resolve_jobject_in_native()

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/conc-unloading-cherry-pick/webrev.00/

Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu