HPCC-12711 Release rowmanager early to prevent leak checking crash

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6732 from ghalliday/issue12711

HPCC-12711 Release rowmanager early to prevent leak checking crash

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>