Multi-process / persistent applications

[mistasse]

For now, only one thread handles all the applications running on a server. Different improvements would be enjoyable :

• It would be convenient to have one thread per session
• It would be convenient to be able to have a process having a webapp, that can modify its state and access its events. The user should be able to disconnect and reconnect and the process should just go on.
  To this end, the computing process could create a thread and a session with a local socket for sharing state and messages, no webserver.
  Another webserver thread could serve the client applications, collect inputs and forward them through the local socket

It seems to me that there is some overlap between those two improvements, and something to do about both at the same time.