Use a worker+redis-based queue to update stats cache

Refs. #276
Set no parents for any Project

children() method of `/projects/` Directory children is not used when
project list is rendered (ProjectSet is used instead).
There is no need to update (cached) stats for `/projects/` Directory.
Init cache for Directory and Store when it is created
Use redis to register TreeItems as dirty

Add is_dirty() method
Provide is_dirty TreeItem attribute to front-end