HPCC-8254 Ensure child datasets are freed up by compound read activities

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #3848 from ghalliday/issue8254

HPCC-8254 Ensure child datasets are freed up by compound read activities

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>