This PR adds a gomatrixserverlib.Filter parameter to functions handling the syncapi_current_room_state table. It does not implement any filtering logic inside the syncapi IncrementalSync/CompleteSync functions, just the APIs for future use.
Default filters are provided as placeholders in IncrementalSync/CompleteSync, so behaviour should be unchanged (except the default 20 event limit)
SQL table will be changed. You can upgrade an existing database using:
```
ALTER TABLE syncapi_current_room_state ADD COLUMN IF NOT EXISTS sender text;
UPDATE syncapi_current_room_state SET sender=(event_json::json->>'sender');
ALTER TABLE syncapi_current_room_state ALTER COLUMN sender SET NOT NULL;
ALTER TABLE syncapi_current_room_state ADD COLUMN IF NOT EXISTS contains_url bool;
UPDATE syncapi_current_room_state SET contains_url=(event_json::json->>'content')::json->>'url' IS NOT NULL;
ALTER TABLE syncapi_current_room_state ALTER COLUMN contains_url SET NOT NULL;
```
Note: This depends on #436 (and includes all its commits). I'm not sure if Github will remove the duplicated commits once #436 is merged.
