Earlier we introduced `BlockManager` to separate data access logic from
underlying file read and write logic.
This CL further unifies all `SegmentV2` data access to the `BlockManager`,
removes the previous `FileManager` class, and move the file cache to the `FileBlockManager`.
There are no logical changes to this CL.
After this CL, all user table data is read through the `WritableBlock` and `ReadableBlock`
returned by the `BlockManager`, and no file operations are performed directly.
8aa8b8c96d