HPCC-7866 - Use CInterfaceOf instead of double inheritance

Saves a few % points of memory

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #4349 from jakesmith/hpcc-7866

HPCC-7866 - Use CInterfaceOf instead of double inheritance

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>