Guarantee that cache is initialized during startup, and only once,
instead of every time a `MetadataHandler` object is instantiated.
Also, improve logic to determine `fixtures` paths.
When retrieving the related screenshot for a `Save` or `State`, we were
retrieving a very heavy representation of the associated `Rom` object,
only to iterate through its screenshots to find the one we needed.
This change modifies the `Save` and `State` models to directly query the
`Screenshot` model, which is much faster and more efficient. The
`DBScreenshotsHandler` has been updated to include a new `filter` method
that will simplify building queries using SQLAlchemy, something we can
extend to other handlers in the future.
Fixes#1925.
