transport: Fix pending dials memory leak (#271)

The `pending_dials` is populated with a connection ID when initiating
outbound connections.
The state is rightfully cleaned up when the dialing fails, and error
propagated back to the user.
However, the state was not cleared when the dialing succeeded. This was
leading to a subtle memory leak in litep2p.

### Testing Done

I used a custom-patched version of litep2p to log the number of pending
dials. After a few hours, the pending dials for both TCP and WebSocket
connections stabilized at just a few.

```
2024-10-22 17:37:56.252  INFO tokio-runtime-worker litep2p::tcp: status pending_dials=1 pending_inbound_connections=0 pending_connections=1 pending_raw_connections=0 opened_raw=0 cancel_futures=0 pending_open=0
2024-10-22 17:38:26.252  INFO tokio-runtime-worker litep2p::tcp: status pending_dials=0 pending_inbound_connections=0 pending_connections=0 pending_raw_connections=1 opened_raw=0 cancel_futures=1 pending_open=0
2024-10-22 17:38:56.253  INFO tokio-runtime-worker litep2p::tcp: status pending_dials=0 pending_inbound_connections=0 pending_connections=0 pending_raw_connections=0 opened_raw=0 cancel_futures=0 pending_open=0
2024-10-22 17:39:26.253  INFO tokio-runtime-worker litep2p::tcp: status pending_dials=0 pending_inbound_connections=0 pending_connections=0 pending_raw_connections=0 opened_raw=0 cancel_futures=0 pending_open=0
2024-10-22 17:39:56.252  INFO tokio-runtime-worker litep2p::tcp: status pending_dials=0 pending_inbound_connections=0 pending_connections=0 pending_raw_connections=0 opened_raw=0 cancel_futures=0 pending_open=0
2024-10-22 17:40:26.252  INFO tokio-runtime-worker litep2p::tcp: status pending_dials=0 pending_inbound_connections=0 pending_connections=0 pending_raw_connections=1 opened_raw=0 cancel_futures=1 pending_open=0
2024-10-22 17:40:56.252  INFO tokio-runtime-worker litep2p::tcp: status pending_dials=0 pending_inbound_connections=0 pending_connections=0 pending_raw_connections=0 opened_raw=0 cancel_futures=0 pending_open=0
2024-10-22 17:41:26.252  INFO tokio-runtime-worker litep2p::tcp: status pending_dials=0 pending_inbound_connections=0 pending_connections=0 pending_raw_connections=0 opened_raw=0 cancel_futures=0 pending_open=0
```

Closes: #269

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

src/transport/tcp/mod.rs

@@ -557,6 +557,7 @@ impl Stream for TcpTransport {
                 Ok(connection) => {
                     let peer = connection.peer();
                     let endpoint = connection.endpoint();
+                    self.pending_dials.remove(&connection.connection_id());
                     self.pending_open.insert(connection.connection_id(), connection);
 
                     return Poll::Ready(Some(TransportEvent::ConnectionEstablished {

src/transport/websocket/mod.rs

@@ -599,6 +599,7 @@ impl Stream for WebSocketTransport {
                 Ok(connection) => {
                     let peer = connection.peer();
                     let endpoint = connection.endpoint();
+                    self.pending_dials.remove(&connection.connection_id());
                     self.pending_open.insert(connection.connection_id(), connection);
 
                     return Poll::Ready(Some(TransportEvent::ConnectionEstablished {