Partition result_storage's file_storage

So, instead of having the result stored in:
`/tmp/thumbor/result_storage/webp/RQqyAmMvcHl7D5XOCvonKvZgync=/`

it's stored in:
`/tmp/thumbor/result_storage/webp/RQ/qy/RQqyAmMvcHl7D5XOCvonKvZgync=/`