New client management

[Christian-Stieber]

The Client/ClientInfo stuff needs some major revamp.

Points that I remember encountering:

• we need a way for clients to terminate automatically when they are not needed anymore. Probably some kind of work-counter that gets increased when something works on the client, and decreased when its done
• we need a simple, proper way to launch a client or use the already running one, and do something on it. Make sure we actually know WHEN we can start doing things!
• think about whether the Executor module is still the way to go to make things happen on the client
• the ClientInfo feels like a random pile of things that were just stuffed in there because they didn't fit into the client and there was no other place

More thoughts:

• each "activity" should probably have a defined, recognizable completion. Ideally, all activities should have blocking APIs, at least optionally.