Remove DataTable V2 code after 0.8.0 release

[mqliang]

As discussed in #6710 (comment), We will stop supporting DataTable V2 after 0.8.0 is released.

Create this issue to track TODOs after 0.8.0 release:

• remove all DataTable V2 related code.
• change the getMetadata() API return a Map<MetadataKey, String> instead of Map<String, String>

[walterddr]

is this ticket closed?

I recently discovered some performance benefit if we can remove V2 support

• reduce float size from 8bytes to 4bytes
• move bytes type to variable byte buffer instead of converting to hex string and using String type (dictionary map) to store data
• some other issues with BYTES_ARRAY and OBJECT type.

Please let me know if you would like to extend this issue, alternatively, I can also create a new issue.