BIND 10 #2383: add dns::Name constructor to be usable from MasterLoader

[BIND 10 Development]

#2383: add dns::Name constructor to be usable from MasterLoader
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |  UnAssigned
            Priority:  medium        |                       Status:  new
           Component:  libdns++      |                    Milestone:  New
           Sensitive:  0             |  Tasks
         Sub-Project:  DNS           |                     Keywords:
Estimated Difficulty:  0             |              Defect Severity:  N/A
         Total Hours:  0             |  Feature Depending on Ticket:
                                     |  loadzone-ng
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 The new constructor's signature would be:

 {{{#!cpp
 Name::Name(const char* name_data, size_t data_len, const Name* origin,
           /* bool downcase? */);
 }}}

 Then `MasterLoader` can pass the string region data and possibly the
 origin when the name is not absolute.  String region could be
 std::string, but constructing it would be expensive, so I propose
 bare data without any copy or object creation here.

 We also need to extend the interpretation of '@' using the origin.

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Ticket URL: <http://bind10.isc.org/ticket/2383>
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