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>
Reinstate scope checking.

Workunit @scope checking was used after all, but still needed
refactoring in the CConstWUIterator, to avoid all workunit
objects being constructed upfront.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>