Parse datasource_id as integer on the server side (apache#6359)

- Datasource ids are being serialized into strings on the client side and python's default json.load doesn't deserialize strings representing integers as integers. Adding an integer parsing step when initializing QueryContext.

superset/common/query_context.py

@@ -19,7 +19,7 @@ def __init__(
             queries: List[Dict],
     ):
         self.datasource = ConnectorRegistry.get_datasource(datasource.get('type'),
-                                                           datasource.get('id'),
+                                                           int(datasource.get('id')),
                                                            db.session)
         self.queries = list(map(lambda query_obj: QueryObject(**query_obj), queries))