zb: ObjectServer to only serve methods destined to its connection

While in most cases, you'd want ObjectServer to take over all incoming method calls on its connection but in certain cases, you don't. For example, if you'd build a bus implementation on top of zbus, you'll likely want to make use of ObjectServer for the Freedesktop standard interfaces required from a bus, while being able to receive messages on the same connections destined for other peers to send them off to the right connection. If ObjectServer would serve all messages, it'll just return an error for all method calls destined for other peers.

I'm sure there are other (not so niche) use cases for this.