8144714: Add extension point to G1 evacuation closures
Reviewed-by: ehelin, jmasa

8135154: Move cards scanned and surviving young words aggregation to G1ParScanThreadStateSet
Reviewed-by: tschatzl, ehelin

8135152: Create a G1ParScanThreadStateSet class for managing G1 GC per thread states
Reviewed-by: tschatzl, ehelin