Motivation:
The current implementation will provide a DnsRawRecord, which, while
containing the host name it resolves to, would require the user to
decode the name using the decode method currently private to
DefaultDnsRecordDecoder, which in fact means copying it.
Modifications:
Introduce DnsPtrRecord, which is a specialization of DnsRecord which
provides a decoded host name.
Result:
PTR Records are much easier to work with, as the name is decoded already.
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