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.

README.md

@@ -367,7 +367,7 @@ file, in that order of precedence. Flags work before or after the subcommand.
 | `--shared-dir` (repeatable)    | `SOULSEEK_SHARED_DIR`               | `shared_dir` / `shared_dirs` | the download dir          |
 | `--listener-port`              | `SOULSEEK_LISTENER_PORT`            | `listener_port`              | `2234`                    |
 | `--no-listener` / `--listener` | `SOULSEEK_NO_LISTENER`              | `disable_listener`           | listener on               |
-| `--max-concurrent-downloads`   | `SOULSEEK_MAX_CONCURRENT_DOWNLOADS` | `max_concurrent_downloads`   | `5`                       |
+| `--max-concurrent-downloads`   | `SOULSEEK_MAX_CONCURRENT_DOWNLOADS` | `max_concurrent_downloads`   | `20`                      |
 | `--search-timeout`             | `SOULSEEK_SEARCH_TIMEOUT`           | `search_timeout`             | `10`                      |
 | `--log-file`                   | `SOULSEEK_LOG_FILE`                 | not a config key             | stderr                    |