Changes needed for old python version

It seems that a particular way to convert namedtuple to doct does not
work on old python versions (<= 2.7.5). Need to use a way which works
across the versions
Partial-Bug: #1659180

Change-Id: Iacf7fc4e0e1e23999734e45baadf950126898f1e

src/opserver/opserver.py

@@ -133,7 +133,7 @@ def redis_query_start(host, port, redis_password, qid, inp, columns):
     if columns is not None:
         for col in columns:
             m = TableSchema(name = col.name, datatype = col.datatype, index = col.index, suffixes = col.suffixes)
-            col_list.append(m.__dict__)
+            col_list.append(m._asdict())
     query_metadata = {}
     query_metadata['enqueue_time'] = OpServerUtils.utc_timestamp_usec()
     redish.hset("QUERY:" + qid, 'query_metadata', json.dumps(query_metadata))