identify: Fix memory leak of unused pending_opens (#273)

The identify protocol implementation leaked SubstreamIds and PeerIds via
the `pending_opens` hashMap.
Objects were only inserted in the `pending_opens`, however they were
never removed.

The only possible purpose of `pending_opens` is to double-check the
events coming from the service layer: `TransportEvent::SubstreamOpened`.
However this is not needed, as illustrated by the current
implementation.

Part of endeavors to fix memory leaks:
paritytech/polkadot-sdk#6149

### Testing Done

- custom patched litep2p to dump the internal state of identify protocol
running in kusama

cc @paritytech/networking

Signed-off-by: Alexandru Vasile <[email protected]>

src/protocol/libp2p/identify.rs

@@ -180,9 +180,6 @@ pub(crate) struct Identify {
     /// Protocols supported by the local node, filled by `Litep2p`.
     protocols: Vec<String>,
 
-    /// Pending outbound substreams.
-    pending_opens: HashMap<SubstreamId, PeerId>,
-
     /// Pending outbound substreams.
     pending_outbound: FuturesUnordered<BoxFuture<'static, crate::Result<IdentifyResponse>>>,
 
@@ -200,7 +197,6 @@ impl Identify {
             public: config.public.expect("public key to be supplied"),
             protocol_version: config.protocol_version,
             user_agent: config.user_agent.unwrap_or(DEFAULT_AGENT.to_string()),
-            pending_opens: HashMap::new(),
             pending_inbound: FuturesUnordered::new(),
             pending_outbound: FuturesUnordered::new(),
             protocols: config.protocols.iter().map(|protocol| protocol.to_string()).collect(),
@@ -211,8 +207,7 @@ impl Identify {
     fn on_connection_established(&mut self, peer: PeerId, endpoint: Endpoint) -> crate::Result<()> {
         tracing::trace!(target: LOG_TARGET, ?peer, ?endpoint, "connection established");
 
-        let substream_id = self.service.open_substream(peer)?;
-        self.pending_opens.insert(substream_id, peer);
+        self.service.open_substream(peer)?;
         self.peers.insert(peer, endpoint);
 
         Ok(())