Add multiserver support with coordinator

[ivancea]

When a server is full (#15), users should be balanced to other servers.

Furthermore, if an user wants to join a specific room (#13), they should try to connect to the server that has that room.

This has many implications:

• Room IDs should be shared between servers in the cluster
• A coordinator node should decide where the user goes, before asking the server itself, to avoid DoS to a single room

Maybe just a coordinator that asks via HTTP if the servers are full, and let the user choose a server? So users would select a server on joining. Servers would discard connections if they are already full.

Or just make it automatic, by having the servers have an endpoint answering with the room ids, rooms limits, and server limit