Avoid the redundat deserialization of certain StepExecution fields before saving to persistent job repository

In JdbcRepository (MongoRepository maybe too), before saving StepExecution fields such as reader checkpoint info, writer checkpoint info, exception and user persistent data, they are deserialized from bytes to object, and then serialized to bytes to be stored (e.g, as BLOB in db).

We should be able to avoid this extra round of deserialization and serialization, by adding methods to StepExecutionImpl to return their serialized form.