WIP feat(fileblock) : Change return format to dict

The dictionary keys are blockids and values are offset in the
form of lists

We need to benchmark this.

The changed return format
```javascript
{
"63cac3ed731e4b1f868cb3adaef591464de54a67": [100, 400]
"a6f76d7c3b9e2576fa74c5dc438304bcdf3247d0": [200, 300]
"fecfd28bbc9345891a66d7c1b8ff46e60192d284": [0]
}
```
Yes, I have begun working on it. I debugged an existing unit test, and indeed I was getting back offsets in ascending order. I wrote this under the assumption that we weren't getting them in order.