respect SftpVersionSelector when establishing a new connection

Even if using an SftpVersionSelector (e.g. created by SftpVersionSelector.fixedVersionSelector), SSHD sends the maximum protocol version that is supported (namely 6) in the initial SSH_FXP_INIT datagram. This behaviour is hard-coded and may cause problems if the server doesn't correctly handle the request/support protocol version 6. One of our customers is using a server that initially accepts protocol version 6, but then fails every single request by sending invalid data. Actually, the server only correctly supports SFTP 3, so this clearly is a bug in the server implementation - but could be circumvented quite easily by starting with protocol version 3. Furthermore an additional (and actually unnecessary) version renegotiation may be required later.

If using an SftpVersionSelector that prefers a certain protocol version other than 6 (custom or created by SftpVersionSelector.preferredVersionSelector), an additional (and actually unnecessary) network round trip is required for protocol version renegotiation. 

Please find attached a minimally invasive patch against the official 2.4.0 source to respect the version(s) accepted by an SftpVersionSelector for choosing the initial protocol version.