Bug: high ram usage during `benji rm`

[FedericoHeichou]

Describe the bug

If you have a big version (like 20 millions of rows), benji will use too much ram while performing the moving of those rows from blocks to deleted_blocks

To Reproduce

Just create a big version (you can just add junk into blocks table for that version) and run benji rm for that version.

Additional context

I will submit a PR