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/client/operations.rs

@@ -471,10 +471,7 @@ impl Client {
                                 let registry = context.peer_registry.clone();
                                 drop(context);
                                 if let Some(registry) = registry {
-                                    let response =
-                                        crate::message::server::MessageFactory::build_transfer_response_message(
-                                            transfer,
-                                        );
+                                    let response = MessageFactory::build_transfer_response_message(transfer);
                                     let _ = registry.send_to_peer(
                                         &username,
                                         PeerMessage::SendMessage(response),