Client notifications and demos

Most of the client notification framework is complete in these commits, which essentially stores a key->list mapping of "interested" clients. Whenever a key is changed, we tell everyone apart from the originator (client who initiated the change) that the key was changed.

We do not tell them the value or layer, they have to retrieve the changed key themselves using normal methods. I've not finished the client side of things to actually receive and deal with CHANGED messages, as I wanted to get the server side (and some libbuxton) stuff in first.

Note I also started on some demos, for benchmarking, etc. An important reason for having this around is to help us implement the client-side notification grabbing easier, by writing demos using 2 BuxtonClients, etc.