Support reservation refinement for hierarchical roles.

With the introduction of hierarchical roles, Mesos provides a mechanism to delegate resources down a hierarchy.
To complement this, we'll introduce a mechanism to refine the reservations down the hierarchy.

For example, given resources allocated to role foo, it can be further reserved for foo/bar.
When the resources allocated to foo/bar is unreserved, it goes back to where it came from. In this case, back to role foo.