APT_JOIN_POOL_SIZE environment variable in DataStage

Set the APT_JOIN_POOL_SIZE environment variable to set the memory limit for the join operator. After the join operator hits the memory limit, the data is written to files on the scratch disk. The files are cleaned up when the join operation is complete. This parameter is not set by default. To enable it, set this environment variable. The variable needs to be set in bytes. The memory size value is used for the upper bound of memory for the join.