HPCC-11968 Port back unique allocator change to 4.2.x

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #6359 from jakesmith/hpcc-11968-4.2.x

HPCC-11968 Port back unique allocator change to 4.2.x

Reviewed-By: Gavin Halliday <gavin.halliday@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>