[Enhancement] allocate blocks in scanner_context on demand and free them timely

[dataroaring]

Search before asking

• I had searched in the issues and found no similar issues.

Description

Currently, _free_blocks is filled in prepare phase in advance, it allocates blocks computed from a config. It consumes a lot memory and is not used for small load.

Solution

• allocate block on demand and limit it as current.
• free them timely, e.g. close

Are you willing to submit PR?

• Yes I am willing to submit a PR!

Code of Conduct

• I agree to follow this project's Code of Conduct

[gitccl]

I'd like to do this task.