Load resources using Resource.load_all_from_stack()

Move responsbility for loading resource information back into the Resource
class. To maintain efficiency, load everything from the database at once in
the new load_all_from_stack() method. This allows us to remove the
_db_resources state from the Stack, collects the implementation in one
place and restores responsibility for its own DB state to the Resource
class.

Change-Id: Ia8059af57131e4b0a986c1f566b78aa935741a23
Merge "Load resources using Resource.load_all_from_stack()"