CommandHandler for the PASV command. Handler logic:
- If the user has not logged in, then reply with 530
- Otherwise, request the Session to switch to passive data connection mode. Return a reply code of 227,
along with response text of the form: "(h1,h2,h3,h4,p1,p2)", where h1..h4 are the
4 bytes of the 32-bit internet host address of the server, and p1..p2 are the 2
bytes of the 16-bit TCP port address of the data connection on the server to which
the client must connect. See RFC959 for more information.