RFR [8]: 2018-06-01, Bulk backports to sh/jdk8

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20180601/webrev.01/

This backports recent work to sh/jdk10:

 [backport] Incorrect label for static heuristics
 [backport] Rename "cancel_concgc" to "cancel_gc"
 [backport] Verifier should dump raw memory around the problematic oops
 [backport] Move heuristics from ShCollectorPolicy to ShHeap
 [backport] Rework GC degradation on allocation failure
 [backport] Rework ClassUnloading* flags handling
 [backport] Check heap stability in C1 WBs
 [backport] ClassUnloadingWithConcurrentMark should be opt-in with Shenandoah
 [backport] More verbose profiling for phase 4 in mark-compact
 [backport] Full GC always comes with liveness data
 [backport] Recycle the regions only once
 [backport] Rename and move ShenandoahPrepareForMarkClosure
 [backport] Reclaim immediate garbage after mark-compact marking
 [backport] Shortcut regions that are known not to be alive
 [backport] Refactor and improve ShenandoahCodeRoots strategies

We are missing the backport that enables ShenandoahCodeRootsStyle=2 by default, because upstream 8u
does not work reliably with it yet. I'll take a look if we can backport some of upstream patches to
upstream 8u to make this work.

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey