SignalStream & PropertyStream shouldn't keep ref to parent Proxy

The following discussion from !408 (merged) should be addressed:

• @danieldg started a discussion: (+3 comments)

  This results in a PropertyStream that can outlive the parent Proxy, which is not what we want because the task that feeds the stream only lives as long as the proxy. I should have been more clear with my prior comment, which was suggesting the prototype be:

      pub async fn receive_property_changed<'p, T>(
          &'p self,
          name: &'p str,
      ) -> PropertyStream<'p, T>