unify: change format for assigning blocks

Its now of the form:
```python
POST /v1.0/vaults/{vault_id}/files/{file_id}
body:

[
    ["fecfd28bbc9345891a66d7c1b8ff46e60192d284", 0],
    ["63cac3ed731e4b1f868cb3adaef591464de54a67", 100],
    ["a6f76d7c3b9e2576fa74c5dc438304bcdf3247d0", 200]
]
```
If the json parser can give you int, these two statement can be merged into:
```
block_id, offset = mapping
```