Fix tracker announce stalls with active scrapes

[szab100]

Fixes #836

rTorrent stops announcing to HTTP trackers while seeding, causing the tracker to eventually mark the torrent as stopped. Experienced through Socks5 proxy (socks5h://) via VPN.

Changes:

• src/tracker/tracker_controller.cc: Use has_active_not_scrape() in receive_success() so the next announce is scheduled as soon as no announce is in flight, regardless of active scrapes
• src/net/curl_stack.cc/h: Add ensure_timeout_scheduled() to guarantee the multi-socket timeout task is armed while any transfer is active, and call it from start_get(), receive_timeout(), and after socket events
• test/torrent/test_tracker_controller.cc: Add regression test test_success_with_active_scrape_reschedules_timeout to verify announce timeout is queued when scrapes are active

Backwards compatibility:
No behavior change for healthy trackers or connections. Stalled HTTP requests through proxy/VPN now time out and retry on schedule, and announces are no longer dropped because of an active scrape.

Announcements work perfectly with the fix.