Return empty result when we get cached values not `from_update`

We can try to get cached_values when:

- getting stats for overview pages;
- calculating stats for VirtualResources (non-cached TreeItems) when
  display overview pages too;
- updating cached values in a RQ jobs;

Let's split these cases with `from_update` parameter.
Fix #293, #294
Use VirtualTreeItem (non-cached) class as a base class for VirtualResource
Change doc strings