Do not traverse the lists by hand, use get_*_by_id functions from discord-util.
This adds get_user_by_id and changes get_channel_by_id to traverse all servers
in case no server is specified.
Get info and keep it in server_info channel list about voice channels. This
might later be used to somehow (imcb_log or nick changes) indicate buddies
being in voice channels (specific or in general).