use table lock while adding new external rpms
    
    PR #794 introduced savepoint-based solution for parallel addition of
    external rpms. In some usecases there could be a lot of external rpms
    being added, so potential for deadlock is much higher than before.
    Locking the table seems to be a bit expensive but SHARE ROW EXCLUSIVE
    should be a working compromise (anyone not writing to the table has
    still read access).
    
    Related: https://pagure.io/koji/issue/3637