HPCC-12191 Reduce calls to createRowAllocator() for roxie NWayMergeJoin

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6424 from ghalliday/issue12191

HPCC-12191 Reduce calls to createRowAllocator() for roxie NWayMergeJoin

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>