Remove slow lookups from beetsplug/aura

It seems like previously filtering by flexible attributes did not work
- I'd receive '{"data": []}' trying to GET `/aura/tracks?filter[play_count]=11`

Now this works, not only for tracks, but for `/aura/artists` and
`/aura/albums` too.

Additionally, this improves `/aura/tracks` response time significantly.
I tried loading the default of 500 tracks from my library:

On `master`, it took ~20s
After this commit, it takes under 1s.

beetsplug/aura.py

@@ -180,8 +180,9 @@ class AURADocument:
                 converter = self.get_attribute_converter(beets_attr)
                 value = converter(value)
                 # Add exact match query to list
-                # Use a slow query so it works with all fields
-                queries.append(MatchQuery(beets_attr, value, fast=False))
+                queries.append(
+                    self.model_cls.field_query(beets_attr, value, MatchQuery)
+                )
         # NOTE: AURA doesn't officially support multiple queries
         return AndQuery(queries)