8144714: Add extension point to G1 evacuation closuresReviewed-by: ehelin, jmasa
8135154: Move cards scanned and surviving young words aggregation to G1ParScanThreadStateSetReviewed-by: tschatzl, ehelin
8135152: Create a G1ParScanThreadStateSet class for managing G1 GC per thread statesReviewed-by: tschatzl, ehelin