HPCC-10022 Allow the max number of persist instances to be set

Set with #option('defaultNumPersistInstances', <n>);

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4988 from ghalliday/issue10022c

HPCC-10022 Allow the max number of persist instances to be set

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>