HPCC-9463 Use CInterfaceOf for a hashtable base class

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4844 from ghalliday/issue9463

HPCC-9463 Use CInterfaceOf for a hashtable base class

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>