Use default upstream resolvers as fallback for system DNS resolution

[0xERR0R]

Name resolution for download links and DoH/DoT/Hostname resolvers should use one of default upstream resolver as fallback:

1. if "bootstrapDns" is defined, use this. If resolution not successful, use 2 as fallback
2. use default system DNS resolver, if the resolution is not successful, use 3 as fallback
3. use one of default upstream resolvers (configuration in config.xml). Use only resolvers defined as IP address

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[ThinkChaos]

I think 1 and 2 were done as part of encrypted bootstrap: #485