Multi-User support

[janbormet]

In the demo, one channel-service needs to serve two neuron wallets. For this, we introduced a hack that uses the network address of the request origin to identify the user that issued the request.

We should employ a proper solution for this.

Also, the current shape of the UserRegister runs into issues, if both parties in a channel are managed by the same channel-service and thus the same UserRegister instance.