What does this implement/fix?

This PR implements TCP lookups for the internal DNS client used to derive names for clients and upstream servers. The reason for adding this was #1975 during which we have found that existing UDP lookups may fail silently because of UDP packet fragmentation/truncation. Using TCP avoids this issue altogether. It can reliably be triggered in development-v6 when setting 8.8.8.8 as upstream server and enabling DNSSEC validation.

While discussing #1975, we found another glitch that causes names of upstream servers to never be resolved when starting with no/an empty database. Only after the first restart of FTL after database initialization, upstream server names are resolved. This is fixed now as well.

Related issue or feature (if applicable): #1975

Pull request in docs with documentation (if applicable): N/A

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pi-hole / FTL

Improve internal DNS resolver #1976

What does this implement/fix?

Checklist: