node: use `outbound.get` over `outbound.get_mut` — heartwood

The following block did not need mutable access to the outbound value.

To avoid confusion around mutable access, the call was downgraded to get so that the reader can safely know there is no mutation happening.

     }
     fn transport_registered(&mut self, token: Token, _transport: &Self::Transport) {
         if let Some(outbound) = self.outbound.get_mut(&token) {
         if let Some(outbound) = self.outbound.get(&token) {
             log::debug!(target: "wire", token=token.0; "Outbound peer resource registered for {}", outbound.nid);
         } else if self.inbound.contains(&token) {
             log::debug!(target: "wire", token=token.0; "Inbound peer resource registered");