message: add handy function to enable subscription from connected clients
diff: @@ -5,7 +5,7 @@ use std::{
  };
  use tokio_io::AsyncWrite;
  -use super::{nt, ConnectionState, LightWeightConnectionState, Message, NodeId};
  +use super::{nt, ConnectionState, LightWeightConnectionState, Message, NodeId, KeepAlive};
  pub type Outbound = Message;
  @@ -70,6 +70,18 @@ impl<T: AsyncWrite> OutboundSink<T> {
  })
  }
  + /// initialize a subscription from the given outbound halve.
  + pub fn subscribe(
  + self,
  + keep_alive: KeepAlive,
  + ) -> impl Future<Item = (nt::LightWeightConnectionId, Self), Error = OutboundError> {
  + self.new_light_connection()
  + .and_then(move |(lwcid, connection)| {
  + connection.send(Message::Subscribe(lwcid, keep_alive))
  + .map(move |connection| (lwcid, connection))
  + })
  + }
  +
  /// close a light connection that has been created with
  /// `new_light_connection`.
  ///
  