add explicit names to these foreign key constraints so they can be more easily referenced by a fixed name.

also add ON DELETE CASCADE to these foreign key constraints to help deleting unneeded data.

Compatibility Note:

Existing jdbc repository database will continue work with its existing schemas, because JBeret batch runtime will only create tables when they do not exist. However, existing batch application should consider altering job repository tables to align with the new DDL.

jberet-db2.ddl already has foreign key constraint names (JOB_EXECUTION_JOB_INSTANCE_FK, STEP_EXECUTION_JOB_EXECUTION_FK, PARTITION_EXECUTION_STEP_EXECUTION_FK), but is updated to new names to be consistent with other ddl files(FK_JOB_EXECUTION_JOB_INSTANCE, FK_STEP_EXECUTION_JOB_EXECUTION, FK_PARTITION_EXECUTION_STEP_EXECUTION).