HPCC-10532 Serialize progress information so backward compatible.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge branch 'issue10532' into candidate-4.2.0

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Community Edition 4.2.0-3 Gold

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>