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/actor/mod.rs

@@ -87,7 +87,7 @@ impl ActorSystem {
     /// The callback receives the actor handle before `on_start` is called
     pub fn spawn_with_handle<A: Actor, F>(
         &self,
-        actor: A,
+        mut actor: A,
         init: F,
     ) -> ActorHandle<A::Message>
     where
@@ -96,7 +96,6 @@ impl ActorSystem {
         let (sender, receiver) = channel::<ActorMessage<A::Message>>();
         let handle = ActorHandle { sender };
         let handle_for_init = handle.clone();
-        let mut actor = actor;
 
         let spawned = thread::Builder::new()
             .name("soulseek-actor".to_string())