Be more helpful when listing peers

[lewisgoddard]

We can list peers more intelligently. Clients just starting a download don't need to know about a neighbour that has 0% as well, and completed downloads don't need to know each other either. Seeders being introduced to peers near completion speeds up conversion and improves ratios.

• If a client has just started their download: list most recent peers with >50% completion.
• If a client has not completed their download: list peers by most completed first, then most recent.
• If a client has completed their download: list peers by most completed first, but exclude 100% completions, then most recent.

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