reduce redundancy in item queries

Now the SELECT query for items only appears in Library.items(). All other
functions that retrieve items go through this method. This should make it
easier to change the way flexattrs are picked up.

beets/library.py

@@ -1314,13 +1314,8 @@ class Library(BaseLibrary):
     def load(self, item, load_id=None):
         if load_id is None:
             load_id = item.id
-
-        with self.transaction() as tx:
-            rows = tx.query('SELECT * FROM items WHERE id=?', (load_id,))
-            flex_rows = tx.query(
-                'SELECT * FROM item_attributes WHERE entity_id=?', (load_id,)
-            )
-        item._fill_record(rows[0], get_flexattrs(flex_rows))
+        stored_item = self.get_item(load_id)
+        item._fill_record(stored_item.record, stored_item.flexattrs)
         item._clear_dirty()
 
     def store(self, item, store_id=None, store_all=False):
@@ -1468,14 +1463,10 @@ class Library(BaseLibrary):
     def get_item(self, id):
         """Fetch an Item by its ID. Returns None if no match is found.
         """
-        with self.transaction() as tx:
-            rows = tx.query("SELECT * FROM items WHERE id=?", (id,))
-            flex_rows = tx.query(
-                'SELECT * FROM item_attributes WHERE entity_id=?', (id,)
-            )
-        if rows:
-            return Item(rows, get_flexattrs(flex_rows))
-        else:
+        items = self.items(MatchQuery('id', id))
+        try:
+            return items.next()
+        except StopIteration:
             return None
 
     def get_album(self, item_or_id):
@@ -1590,12 +1581,7 @@ class Album(BaseAlbum):
         """Returns an iterable over the items associated with this
         album.
         """
-        with self._library.transaction() as tx:
-            rows = tx.query(
-                'SELECT * FROM items WHERE album_id=?',
-                (self.id,)
-            )
-        return ResultIterator(rows, self._library)
+        return self._library.items(MatchQuery('album_id', self.id))
 
     def remove(self, delete=False, with_items=True):
         """Removes this album and all its associated items from the