RFR [8], 2018-08-24, Bulk backports to sh/jdk8

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20180824/webrev.01/

This backports recent work to sh/jdk11:

 [backport] Comprehensible GC trigger logging
 [backport] Fix ShHeap::notify_alloc usages: it accepts words, not bytes
 [backport] Adaptive/Traversal heuristics rewrite for allocation rate
 [backport] Remove ShHeuristics::print_threshold
 [backport] Avoid using uintx in ShenandoahHeapRegion
 [backport] shenandoah_assert_correct should check object/forwardee klasses
 [backport] Evac assist should touch marked objects only

Testing: hotspot_tier3_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey