Rework NETINFO cell parsing and generation with trunnel

In channel_tls_process_netinfo_cell() we have:

1720   /* Decode the cell. */
1721   timestamp = ntohl(get_uint32(cell->payload));
1722   if (labs(now - chan->conn->handshake_state->sent_versions_at) < 180) {
1723     apparent_skew = now - timestamp;
1724   }
1725 
1726   my_addr_type = (uint8_t) cell->payload[4];
1727   my_addr_len = (uint8_t) cell->payload[5];
1728   my_addr_ptr = (uint8_t*) cell->payload + 6;
1729   end = cell->payload + CELL_PAYLOAD_SIZE;
1730   cp = cell->payload + 6 + my_addr_len;

and in connection_or_send_netinfo() we write the cell to connection buffer. Trunnel should be used.