Add content and snapshot store references

Update database object to hold reference to the
data stores.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

metadata/content_test.go

@@ -23,7 +23,7 @@ func createContentStore(ctx context.Context, root string) (content.Store, func()
 		return nil, nil, err
 	}
 
-	return NewContentStore(db, cs), func() error {
+	return NewContentStore(NewDB(db), cs), func() error {
 		return db.Close()
 	}, nil
 }