RFR [8u], 2018-04-12, Bulk backports to sh/jdk8u

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20180412/webrev.01/

These include observability and obvious bug fixes for our sh/jdk8u adopters:

8aaafef81fa3: [backport] Immediate garbage ratio should not go over 100%
8f2338305348: [backport] Introduce assert_in_correct_region to verify object is in correct region
19c01678e365: [backport] FreeSet should report its internal state before/after GC cycle
05d109967a80: [backport] Disambiguate "upgrade to Full GC" GCause
626d278bcafd: [backport] Report how much we have failed to allocate during Allocation Failure
cd9e28fa0f27: [backport] Use/sort (cached) RegionData not ShenandoahHeapRegionSet (infrastructure)

Testing: hotspot_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey