G1: Introduce API to run multiple separate tasks in a single gangtask

JDK-8214237 is going to join different somewhat independent (sub-)tasks of the post evacuation phase into basically two gangtasks to improve performance (reduce startup and shutdown of work gangs).

To facilitate doing that, introduce an API that manages subtasks of a given gangtask and provide some helpers to do common work.

This CR is only about adding the API and testing to somewhat simplify the actual move of the post evacuate tasks into parallel tasks.