swarm/: Limit negotiating inbound substreams per connection (#2697)

This limit is shared across all `ConnectionHandler`s on a single connection. It
only enforces a limit on the number of negotiating substreams. Once negotiated a
`ConnectionHandler` manages the lifecycle of the substream and has to enforce
limits themselves.

swarm/src/lib.rs

@@ -1364,6 +1364,14 @@ where
         self
     }
 
+    /// The maximum number of inbound streams concurrently negotiating on a connection.
+    ///
+    /// See [`PoolConfig::with_max_negotiating_inbound_streams`].
+    pub fn max_negotiating_inbound_streams(mut self, v: usize) -> Self {
+        self.pool_config = self.pool_config.with_max_negotiating_inbound_streams(v);
+        self
+    }
+
     /// Builds a `Swarm` with the current configuration.
     pub fn build(mut self) -> Swarm<TBehaviour> {
         let supported_protocols = self