Get the rooms guests from socket.io

[severo]

Currently, the list of users in each room is maintained in a global variable, and modified on every user-related event (connect, disconnect, join...). It would be safer to get this information directly from socket.io API, but it doesn't seem to be straightforward (see https://stackoverflow.com/questions/6563885/socket-io-how-do-i-get-a-list-of-connected-sockets-clients - there is a different solution for every major and minor version of socket.io)