HPCC-12679 Ensure statistics cache is cleared on workunit reload

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6734 from ghalliday/issue12679

HPCC-12679 Ensure statistics cache is cleared on workunit reload

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>