schemachange app: Limit outstanding txn in table load.
    Replace progress timeout assert with an error message.
    Add progress reports to table load.
    Add debugging msgs to table load.
    nb. Table load is quite slow. The runapp-recover test does cluster recovery during this time.
    This is mostly counterproductive. Should investigate way to coordinate the app and apprunner
    to concentrate recovery testing on the schemachange operations.