lib/config, lib/discover: Support new discovery cluster (ref #4618)

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This adds one new feature, that discovery servers can have ?nolookup to
be used only for announces. The default set of discovery servers is
changed to:

- discovery.s.n used for lookups. This is dual stack load balanced over
  all discovery servers, and returns both IPv4 and IPV6 results when they
  exist.

- discovery-v4.s.n used for announces. This has IPv4 addresses only and
  the discovery servers will update the unspecified address with the IPv4
  source address, as usual.

- discovery-v6.s.n which is exactly the same for IPv6.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4647

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