HPCC-8729 - Improve workunit iterator efficiency

Delay creating workunit objects until needed
Also remove some odd unused workunit @scope authentication

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #4047 from jakesmith/hpcc-8729

HPCC-8729 - Improve workunit iterator efficiency

Reviewed-By: Gavin Halliday <gavin.halliday@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>