Raid introduce the new dependency between blocks within a file.
The blocks help decode each other. Therefore we should avoid put them on the same machine.
The proposed BlockPlacementPolicy does the following
1. When writing parity blocks, it avoid the parity blocks and source blocks sit together.
2. When reducing replication number, it deletes the blocks that sits with other dependent blocks.
3. It does not change the way we write normal files. It only has different behavior when processing raid files.