Create common ResourceUsage class to track labeled resource usages in Capacity Scheduler

Since we have labels on nodes, so we need to track resource usage by labels, includes

• AM resource (to enforce max-am-resource-by-label after YARN-2637)
• Used resource (includes AM resource usage)
• Reserved resource
• Pending resource
• Headroom

Benefits to have such a common class are:

• Reuse lots of code in different places (Queue/App/User), better maintainability and readability.
• Can make fine-grained locking (e.g. accessing used resource in a queue doesn't need lock a queue)