Change the tree stat caching so that in routine use we only need to do updates
on the owning tree, not read the db. Do this by using the change in size
known via the cfs_file object.

Add method to generate the tree stats from the database and one to do it over
all file groups (in case things get out of sync somehow).
Add method to generate all of the tree stats of a specific root cfs directory
from the database.
Use instance method for updating stats from the database for one root to implement
the class method for doing all of them.