[improvement] Simplify scan node logic: removing _materialized_blocks…

[jacktengg]

… and transfer thread

Proposed changes

Issue Number: close #xxx

Problem Summary:

Describe the overview of changes.

Checklist(Required)

1. Does it affect the original behavior: (Yes/No/I Don't know)
2. Has unit tests been added: (Yes/No/No Need)
3. Has document been added or modified: (Yes/No/No Need)
4. Does it need to update dependencies: (Yes/No)
5. Are there any changes that cannot be rolled back: (Yes/No)

Further comments

If this is a relatively large or complex change, kick off the discussion at [email protected] by explaining why you chose the solution you did and what alternatives you considered, etc...

[morningman]

Could you please explain more about the threading model of this olap scan node. For example:

1. How to determine the concurrency.
2. How to submit the scanner thread.
3. How to transfer the block from scanner to scan node.

So that other developer can easily understand its logic