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>
HPCC-10910 Ensure abort is checked before spawning a child

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>