bugfix: migration of dataprep entities

[joohokim1]

Describe the bug
The following problems occur when migrating entities related to dataprep.

1. When migrating the dataflow-dataset relation table, if the dataset or dataflow entity is missing, you will get a foreign key constraint exception.
2. When there's CLOB columns in the original entity, you'll get a cannot cast exception. (For MySQL it works fine, but for H2 it's not)

Dataprep 관련 entity들을 migrate하는 데에 다음의 문제들이 발생합니다.

1. 존재하지 않는 dataset 또는 dataflow간의 관계 table migration이 foreign key constraint exception 발생
2. CLOB column을 옮길 때에 String으로 casting하다가 발생하는 문제 (MySQL은 제대로 동작. H2는 에러 발생)

To Reproduce
Steps to reproduce the behavior:

1. Startup with setting polaris.dataprep.pmonInterval: 30
2. Bootup fails if you use H2 as the meta DB.
3. If there are some rows in prep_dataflow_dataset that corresponding dataflow or dataset is missing, you'll see error log, every pmonInterval seconds.

Expected behavior

1. Startup should succeed with H2 meta DB.
2. Only relations with existing foreign keys should be migrated.

Screenshots

Desktop (please complete the following information):

• OS: MacOS
• Browser: Chrome
• Version: 68.0.3440.106