RFR: Hook up GCLABs to Elastic LAB support

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/elastic-gclabs/webrev.01/

Eyeballed Visualizer on some workloads, and noticed that after we allowed GCLABs to take the entire
region (as the side effect for elastic TLABs to have max_tlab_size = heap_region_size), they started
to fragment the heap aggressively. Thus we need to resolve the TODO item to hook them up to elastic
machinery right away.

Testing: tier3_gc_shenandoah, benchmarks

Thanks,
-Aleksey