Summary:
- Replace null with root() to represent root container for consistency
- Support rcm inheritance by Thread.inheritedResourceContainer

Test Plan: jtreg com/alibaba/rcm/ multi-tenent/

Reviewed-by: luchsh, tanghaoth90

Issue: https://github.com/alibaba/dragonwell8/issues/95

Summary:
Introduce ResourceContainer to manage resources.
ResourceContainer defines a set of resource Constraint
that limit resource usage by threads.

Test Plan: jtreg com/alibaba/rcm

Reviewed-by: luchsh, sanhong

Issue: alibaba/dragonwell8#87