network/wireguard: cleanups for resolving endpoints

network/wireguard: cleanups for resolving endpoints

This makes
- drop peers_with_unresolved_endpoint and peers_with_failed_endpoint,
- drop destroy handler for sd_resolve_query, and manage each query by peer,
- add random fluctuation to the timeout for retry handler,
- retry timer event source is now managed by peer,
- use sd_event_source_disable_unref().