dbus: methods return chan, do not block

Allow callers to ignore the result of jobs corresponding to unit methods by returning the response channel. Doing this allows fleet to stop caring about the result of jobs and work around https://github.com/coreos/fleet/issues/903.
it means we could not parse the ID from dbus, but we are still going to wait on a JobRemoved signal for the returned path. I could just not try to the last path element as an int and this problem sorta goes away. It could still return an unexpected value, though...