FTP request opcode

This PR do:

1. Add req_opcode for returning request opcode in response messages.
2. Offsets response opcodes from request. This change allow us to add new opcodes without compatibility breaks.
3. Remove extra CRC32.
The header needs to be as small as possible in order to get as many bytes for data as we can. Downloading a file in as few packets as possible is critical. Given that I don't think reserving space for future changes is good in this case.