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/src/cli.rs

@@ -275,7 +275,8 @@ pub struct ServeArgs {
     #[arg(long, action = ArgAction::SetTrue)]
     pub wishlist: bool,
 
-    /// How many uploads to run at once; the rest queue, privileged users first
+    /// How many uploads to run at once [default: 10]; the rest queue,
+    /// privileged users first
     #[arg(long, value_name = "N", value_parser = clap::value_parser!(u8).range(1..=64))]
     pub upload_slots: Option<u8>,
 }