[migration] fix backward compat for rename_relation

[datafeed] make cnxset handling of datafeed source more robust currently we may run in some cases where the session has no more cnxset depending on errors and parser's handling of the cnxset. Also, free the cnxset and reacquire it later, letting a chance to other threads to run.

don't use rset.rowcount in predicates, this makes them usable with other types such as lists