HPCC-7928 Ensure each branch of a SEQUENTIAL is independent

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4145 from ghalliday/issue7928

HPCC-7928 Ensure each branch of a SEQUENTIAL is independent

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>