refactor: simplify DNS discovery fallback

[xdustinface]

Use a tokio::time::Interval in the maintenance loop's select! instead of manual SystemTime elapsed tracking.
The interval arm is disabled in exclusive mode via a select guard and skips DNS if peers are already enough.

Also, the DNS lookup branch doesn't try to connect immediately but instead in the next loop cycle, which happens straight away.

Based on:

• fix: make use of stored peers #422

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