Add multi DNS support

[flobernd]

DNS resolution sometimes returns multiple IPs for a single hostname. This is mainly used by cloud instances for load-balancing reasons (DNS round-robin).

Work Items

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1. Implement DnsNodePool
2. DnsNodePool Takes a single hostname and internally resolves to the actual IPs
3. DnsNodePool Might derive from SniffingNodePool, if possible
4. StickyDnsNodePool Takes a single hostname and internally resolves to the actual IPs
5. StickyDnsNodePool Might derive from StickySniffingNodePool, if possible
6. Derive CloudNodePool from DnsNodePool and enable multi DNS by default

Similar to traditional sniffing, the DnsNodePool classes should periodically re-resolve the DNS hostname to refresh the list of node IPs.