Now supporting folder heirarchies. The client will now descend looking for subfolders. This task now opens up multiple outstanding requests to the Engine as well as exercises the database schema.
Closing this ticket opens the door to finishing #3692.
This commit introduces lazy loading of folder contents, which allows the Engine to report back email in chunks rather than all at once (which might require a round-trip to the server). This allows for the local database results to be returned to the caller right away while background I/O is occuring.
