HPCC-9551 Allow Roxie to set thread priorities

Previous fix here was not sufficient

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #5458 from richardkchapman/roxie-threadpriorities

HPCC-9551 Allow Roxie to set thread priorities

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>