improve txn test index handling
    
    Sometimes it is difficult to determine which indexes actually have files,
    and the txn thread would keep looping forever looking for missing files.
    This changes the txn thread to retry a few times, then just start skipping
    missing indexes.  The txn thread will log which indexes it uses and which
    it skipped.
    (cherry picked from 36180b1754e9907a189afff4ed7683ce5ca6b48b)