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/persist/config.rs

@@ -246,7 +246,13 @@ pub struct Resolved {
 
 pub const DEFAULT_SERVER: &str = "server.slsknet.org:2416";
 pub const DEFAULT_LISTENER_PORT: u16 = 2234;
-pub const DEFAULT_MAX_CONCURRENT_DOWNLOADS: usize = 5;
+/// How many downloads run at once by default.
+///
+/// Downstream is the abundant direction, and a Soulseek transfer is paced by
+/// the sending peer — usually a few hundred KiB/s — so filling a modern link
+/// takes many transfers at once, not faster ones. Twenty at a typical peer rate
+/// is tens of MiB/s, and costs twenty threads and sockets, which is nothing.
+pub const DEFAULT_MAX_CONCURRENT_DOWNLOADS: usize = 20;
 pub const DEFAULT_SEARCH_TIMEOUT: u64 = 10;
 
 /// Layer CLI/env values over the config file over defaults.