Revisit o.a.h.y.s.rm.scheduler.ResourceUsage

YARN-3092 introduced ResourceUsage class to track a number of things around resources. The naming is a little ambiguous and hence less conducive to being used elsewhere.

1. UsageByLabel doesn't need to be label specific. A more descriptive name (TenantResourceTracker) might be more apt - since the class tracks more than just usage.
2. Accordingly, ResourceUsage itself can be renamed to more descriptive (and less ambiguous) to LabelWiseTenantResourceTracker or some such.
3. TenantResourceTracker (previously UsageByLabel) should probably be a class on its own, and the private ResourceType should be part of it instead of the mapping against labels.
4. Ideally, would like for the names to say allocation to capture allocation instead of usage.