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]
}
```
The blocks are returned in ascending offset order, so can't we just capture the max offset here? This way we avoid making another expensive pass down below to find the max.  Thoughts? 

I'm thinking...
max_offset = max(max_offset, record[1])) 

We could theoretically just say:

max_offset = record[1]

...but I would want a unit test to ensure that create_file_block_generator always returns with offsets in ascending order.