RFR [8]: 2018-11-04, Bulk backports to sh/jdk8

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20181104/webrev.01/

Recent improvements and bugfixes:

 [backport] Handle metadata induced GC
 [backport] Adjust metaspace size during each GC
 [backport] Remove fix-roots pass in ShHeap
 [backport] Avoid write barrier during full GC
 [backport] Pre-evac JVMTI roots + fixes
 [backport] Protect risky conversion in ShenandoahHeap::millis_since_last_gc
 [backport] Purge unnecessary time conversion in ShenandoahPhaseTimings::record_phase_time
 [backport] Make heuristics tell if we can process references or unload classes
 [backport] Factor out implicit/explicit GC requests
 [backport] Concurrent cycle by default on any external GC request
 [backport] Initial committed regions count can be more than max

Testing: hotspot_tier3_gc_shenandoah {fastdebug|release}

Thanks,
-Aleksey