Update TorrentChecker

[egbertbouman]

This PR does the following:

• Updates DownloadManager.get_metainfo to spend more time in the swarm when doing a health check. It's currently set to 4 minutes. As health checks now potentially take a lot more time, I've increased the maximum simultaneous checks to 5,
• Splits health checks in 2 parts: checking trackers, and if that doesn't yield any results, join the swarm for 4 minutes.
• Removes FakeDHTSession/FakeBep33DHTSession as I think it makes more sense to call get_metainfo separately (as a fallback for torrents without a working tracker).
• Disables checking random trackers, as this only does a partial check. I didn't remove the underlying code, in case we decide to turn this back on in the future.
• Adds a debug panel showing completed health checks.

[egbertbouman]

validate