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

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20171016/webrev.01/

This change backports most of the outstanding work to sh/jdk8u. This is similar to pending backport
to sh/jdk9, but for 8u.

Changes include:

rev 10139 : [backport] Remove ShenandoahGCWorkerPerJavaThread flag
rev 10140 : [backport] Double check for UseShenandoahGC in WB expand
rev 10141 : [backport] Verifier should print verification label at liveness verification
rev 10142 : [backport] Mark bitmap slices commit/uncommit + Aggregated bitmap slicing
rev 10143 : [backport] Fix (external) heap iteration + TestHeapDump should unlock aggressive heuristics
rev 10144 : [backport] Pauses that do not affect heap occupancy should not report heap
rev 10145 : [backport] Make sure -XX:+ShenandoahVerify comes first in the tests
rev 10146 : [backport] Add -XX:+ShenandoahVerify to more interesting tests
rev 10147 : [backport] Clean up dead code

I had to rewrite the runner for TestHeapDump JVMTI test to make it runnable in 8u. Also,
ShenandoahHeap needed touchups to implement oop_iterate properly.

Again, I need sign-offs from everybody for this one too.

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey