Introduce a new message `sendaddrv2` to signal support for ADDRv2.
Send the new message immediately after sending the `VERACK` message.
Add support for receiving and parsing ADDRv2 messages.
Send ADDRv2 messages (instead of ADDR) to a peer if he has
advertised support for it.
Co-authored-by: Carl Dong <contact@carldong.me>
