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.
I renamed the padding in the reserved as to leave room for crc32, for further development. So reserved is padding[2] + uint32_t.

Maybe better is to remove this space and leave padding only.