Unnecessary lifetime in `SignalStream<'a>` prevents moving into spawned tasks.

The SignalStream<'a> has a lifetime that isn't actually used:

#[derive(Debug)]
pub struct SignalStream<'a> {
    stream: Join<MessageStream, Option<MessageStream>>,
    src_unique_name: Option<UniqueName<'static>>,
    phantom: PhantomData<&'a ()>,
}

I imagine this was added to prevent the stream from outliving the proxy object. However, a Stream already has an interface for dealing with that situation: it can return Ready(None) from poll_next() when it will no longer produce any value.

The lifetime now prevents moving the stream into a spawned task. As a result, all handling of signals must now happen in the same task where the connection was made. This is quite a big restriction, and it seems artificial in the case of a SignalStream.