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.

benchmarks/stress-baseline.json

@@ -1,11 +1,11 @@
 {
-  "score": 97.39,
+  "score": 97.78,
   "parts": {
     "downloads": 1.0000,
     "uploads": 1.0000,
     "searches": 1.0000,
     "browses": 1.0000,
-    "throughput": 0.8257
+    "throughput": 0.8522
   },
   "load": {
     "peers": 1024,