Add a max_sessions option?

[vincentfretin]

I open this for discussion. This is a point I listed in #76

Since #74 we don't count the users connected via the subscribe message anymore for the max_ccu limit (previously based on sessions and not publishers).
To not explode a server with high number of sessions, does it make sense to add another option max_sessions to check the number of sessions?
It may be better to add an api join as listener (not publisher), add a list of listeners struct in the switchboard to be able to count them as users for the max_ccu. max_room_size would still be max publishers in a room, it shouldn't count listeners.

[vincentfretin]

Of course one would first tune the event_loops param in janus.jcfg to handle lots of sessions.
It seems hubs used event_loops = 128 https://github.com/mozilla/hubs-ops/blob/4a353b156f123262026af3450e8f91b60f9709f5/plans/janus-gateway/habitat/config/janus.cfg#L7
But even with that, the service may be degraded and the audio may become choppy if there are too many sessions on a server I guess?
I don't currently have lots of users and didn't do any load testing yet, so I didn't tune event_loops for now.