perf: raise transfer defaults and stop uploads sleeping on their slot

The slot cap merged from master defaults to 2 uploads, and the CLI capped
downloads at 5. Those are the numbers you pick when your own bandwidth is the
scarce resource, but a Soulseek transfer is paced by the peer on the other end
— usually a few hundred KiB/s — so what fills a modern link is many transfers
at once, not faster ones. Upload slots go to 10, downloads to 20. The queue
itself stays: privileged users being able to jump it is a condition of being a
tolerated third-party client.

serve_file also slept half a second after the last byte so the downloader could
drain before the socket closed. With a cap that is time no other peer can use,
and 64 uploads over 10 slots paid it six times over. A half-close says "that is
the whole file" and TCP still delivers everything written before the FIN.

Measured with 64 waiting peers: 2 slots 24.2s, 8 slots 6.2s, 32 slots 3.1s; at
10 slots the linger alone was 5.4s to 3.05s.

soulseek-rs-lib/src/peer/upload_peer.rs

@@ -83,8 +83,11 @@ pub fn serve_file(
     }
     stream.flush()?;
 
-    // Linger so the downloader drains everything before the socket closes.
-    std::thread::sleep(Duration::from_millis(500));
+    // Half-close instead of lingering: the FIN tells the downloader it has the
+    // whole file, and TCP still delivers everything written before it. Sleeping
+    // here held an upload slot for an extra half-second per transfer, which with
+    // a slot cap is time no other peer can use.
+    stream.shutdown(std::net::Shutdown::Write).ok();
     trace!("[upload] served {} to {}:{}", path.display(), host, port);
     Ok(())
 }