make next_client_id a lambda

This is neater at the call site and makes it clear that this is only
ever supposed to be an incremental ID and nothing else.

As suggested by max (pr2502) in
pr2502#66 (comment)

src/server.rs

@@ -12,13 +12,14 @@ use crate::socketwrapper::Listener;
 pub async fn run(config: &Config) -> Result<()> {
     let instance_map = InstanceMap::new(config).await;
     let next_client_id = AtomicUsize::new(0);
+    let next_client_id = || next_client_id.fetch_add(1, Ordering::Relaxed);
 
     let listener = Listener::bind(&config.listen).await
     .context("listen")?;
     loop {
         match listener.accept().await {
             Ok((socket, _addr)) => {
-                let client_id = next_client_id.fetch_add(1, Ordering::Relaxed);
+                let client_id = next_client_id();
                 let instance_map = instance_map.clone();
 
                 task::spawn(