HPCC-9169 Rename SHUFFLE (a temporary name) to subsort

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4288 from ghalliday/issue9169

HPCC-9169 Rename SHUFFLE (a temporary name) to subsort

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>