HPCC-10971 Modify serialization interfaces to allow more efficient spill

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5590 from ghalliday/issue10971

HPCC-10971 Modify serialization interfaces to allow more efficient spill

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>