[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

[c-c db-check] don't skip is/is_instance_of mandatory relation to avoid telling two opposite messages when the relation is missing. Closes #2465659

[ui tabs] avoid duplicate rset argument if already specified in tab kwargs. Closes #2464786