Support TIME command

Sometimes when we need to use a command such as HEXPIREAT or EXPIREAT  to set data will expire at a certain time , due we use of multiple clients, and each client's local time may be inconsistent with others, this will lead to some anomalies occur. Therefore, we need to obtain a standard time from the ledis server to deal with such problems . 
Also, because the client's time may not match with the ledis server,sometimes the data expiration time on the server may be higher or delayed than expected in advance . 
So i suggest to add a TIME command used to get the standard time from server (like redis).