Modify IMLoginManager avoid duplicate server ip address be store in list

If we use List<String> for server ip address, each time we get server address list from the response  we will add the server ip address to the list. 
We did not check if the ip is duplicated or not.

I made a change to choose server ip address based on current timestamp, this is not a well solution.
Maybe we should choose server based on hash of userid or username.