Asynchronous callback

Precautions

1. After receiving the asynchronous notification, you must respond with an 
 httpResponse status code of 200. All other status codes are regarded as callback
 failures. After the callback fails, please go to the merchant's backend 
 tofill the order or call the order filling interface to fill the order.
2. The merchant must verify the x-api-signature of the callback notification,
 where the time window offset between x-api-timestamp and the current timestamp 
 needs to be less than 5s.

Callback request header parameter description

Name	Type	Description
x-api-key	string	appkey
x-api-nonce	string	random string
x-api-timestamp	int(10)	Unix timestamp
x-api-signature	string	Signature Hash value

Callback request body parameter description

Name	Type	Description
apikey	string
sys_trade_no	string	System order number
out_trade_no	string	Merchant order number
assets	string	Assets Type
quantity	decimal(16,6)	Quantity of Assets
exc_rate	decimal(16,6)	Assets exchange rate
usdt_quantity	decimal(16,6)	Paymnet quantity (usdt)
rate	decimal(16,6)	Percentage fee（usdt）
imm_rate	decimal(16,6)	Fixed fee per transaction（usdt）
amount	decimal(16,6)	Actual transfer quantity as same as 'usdt_quantity'（usdt）
fee	decimal(16,6)	handling fee（usdt）
to	string	Receiving address
status	int(1)	Order Status
create_time	int(10)	order time
payfor_time	int(10)	Payment time
callbk_time	int(10)	callback time