8227089: Add timing information for merging humongous remembered sets Reviewed-by: sangheki, kbarrett

8227084: Add timing information for merge heap root preparation Reviewed-by: sangheki, kbarrett

8227671: G1: assert_used_and_recalculate_used_equal performs work in product builds Reviewed-by: kbarrett, sangheki

8226232: Move merge heap roots code out from G1RemSetScanState Reviewed-by: sangheki, kbarrett