8087323: Unify and split the work gang classes
Reviewed-by: jmasa, sjohanss

8081682: AbstractWorkGang::_terminate is never used
Reviewed-by: kbarrett, pliden, brutisso

8080879: Remove FlexibleWorkGang::set_for_termination
Reviewed-by: brutisso, kbarrett, pliden

8079792: GC directory structure cleanup
Reviewed-by: brutisso, stefank, david