[TASK] refactor converters

- move converting logic to appropriate classes
- rename rest_to_lib_transaction_converter to rest-to-tx-converter
- refactor method signatures and working to be consistent
- add unit tests for converters
In the cases where there's a possible confusion between the `async.series` or `async.waterfall` callbacks, I'll keep `async_callback`, otherwise `callback`. As we move forward to replacing `async` with `Promises` this should become less of an issue