HPCC-9965 Add a ,UNSTABLE flag to a JOIN

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #5009 from ghalliday/issue9965

HPCC-9965 Add a ,UNSTABLE flag to a JOIN

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>