Refactor and refine the resource usage API implementation

[achimnol]

We have a new resource usage summarization API to get the usage by project/session/kernel for given periods in #962.

However, we need to refactor the implementation.

• Avoid including unnecessary information in some outputs. For example, the per-project resource usage currently includes metadata about a session and kernel. Why??
  • In the code, all ProjectResourceUsage, SessionResourceUsage, and KernelResourceUsage inherits BaseResourceUsageGroup. (Here, the name "Group" is not related to the concept of projects.)
  • BaseResourceUsageGroup contains direct references to a project, a session, and a kernel object. All subclasses merge the result of to_json() method of the superclass, and this incurs unnecessary inclusion of garbage information.
  • Let's rewrite BaseResourceUsageGroup as a simpler abstract base class like ResourceUsageAggregator with just a few methods (no attributes) like get_total_usage().
  • Just refer this abstract base class when calculating the total usage.
• General tips
  • When defining base classes and subclasses to share the type while differentiating the concrete implementations, do NOT hesitate to make empty classes or declaring just a few abstract methods. It's totally fine. Avoid putting too much subclass-specifics in the base classes.
• Rename/expand some fields.
  • ResourceUsage.nfs: Currently it contains the list of host directory paths of mounted vfolders in the given resource set. "nfs" is a too implementation-specific name. We should relax the concept, like ResourceUsage.vfolder_host_paths.
    • I think we need to add ResourceUsage.volumes to include the set of vfolder hosts (storage volumes) used by the given resource set.