TAJO-2146: Fragment interface cleanup

[jihoonson]

No description provided.

[blrunner]

Could you rebase it?

[jihoonson]

Not ready for review.

[jihoonson]

I don't know why the changes in already committed patches are also shown.
The key changes are

• Recaped Fragment to have a consistent interface.
• Added FragmentSerdeHelper for user-defined fragment serialization and deserialization
• Changed fragment configuration properties in storage-default.xml. A fragment and a fragment serde helper are specified for each tablespace like below.

<property>
  <name>tajo.storage.fragment.kind.file</name>
  <value>org.apache.tajo.storage.fragment.FileFragment</value>
</property>
<property>
  <name>tajo.storage.fragment.serde-helper.file</name>
  <value>org.apache.tajo.storage.fragment.FileFragmentSerdeHelper</value>
</property>

• Removed RowFile.

[jihoonson]

Now, ready for review.

[hyunsik]

IMO, FragmentSerde seems to be a more proper name. As far as I know, Helper is mostly used for a utility class.

[hyunsik]

+1
The patch looks good to me. It's a nice cleanup for the fragment and its related ones. I left a trivial comment. If you agree, you can commit after reflecting the comment.

[jihoonson]

Thanks. I've changed the class name as you suggested.