dbus: overhaul job management

Here's the godbus portion of the problem: https://github.com/godbus/dbus/blob/master/conn.go#L239

And to clarify, we won't have actually run into this specific problem before https://github.com/godbus/dbus/pull/7. When the buffered channel used to fill up, signals were just getting dropped on the floor. We decided that dropping signals silently was incorrect, so we changed that behavior. Now that godbus doesn't just drop signals, the chance for this deadlock exists.