Add session handling

[edgarmueller]

Each session would need its own ResourceSet/EditingContext in order to propery support commands

[planger]

I think we need to first define whether we make it a rule that one modelserver only serves one user (always and forever ;)) and specify the use cases that we'd like to cover. If our usecases don't require a session, we can close this issue without an implementation, but I think it is worth discussing and defining it in the course of this issue, which is why I set it to prio high.