FTP: Add req_opcode field for return request opcode in response message.
FTP: Make responses start from opcode 128.
FTP: Remove CRC32 from protocol.

Extra crc not needed because mavlink already has crc16.
FTP: remove reserved uint32 from header.
FTP: Update unit test for new header size.

_list_test failed.
FTP: remove padding zeroing.
Merge pull request #1357 from vooon/ftp_opcode

FTP request opcode