data source v2 API improvement

Data source V2 is out for a while, see the SPIP here. We have already migrated most of the built-in streaming data sources to the V2 API, and the file source migration is in progress. During the migration, we found several problems and want to address them before we stabilize the V2 API.

To solve these problems, we need to separate responsibilities in the data source v2 API, isolate the stateull part of the API, think of better naming of some interfaces. Details please see the attached google doc: https://docs.google.com/document/d/1DDXCTCrup4bKWByTalkXWgavcPdvur8a4eEu8x1BzPM/edit?usp=sharing