BIND 10 #1604: reorganize RRset class hierarchy to allow further optimization

[BIND 10 Development]

#1604: reorganize RRset class hierarchy to allow further optimization
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |  UnAssigned
            Priority:  major         |                       Status:  new
           Component:  libdns++      |                    Milestone:  Year
           Sensitive:  0             |  3 Task Backlog
         Sub-Project:  DNS           |                     Keywords:
Estimated Difficulty:  0             |              Defect Severity:  N/A
         Total Hours:  0             |  Feature Depending on Ticket:  auth
                                     |  performance
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 This is a subtask for "Pre-establish shortcuts for additional section
 processing" described in
 https://lists.isc.org/pipermail/bind10-dev/2012-January/002985.html

 We eventually need to introduce a specialized RRset used within the
 in memory data source that knows internal details of the data source
 and holds shortcut information to provide additional information of
 the RRset efficiently.

 To do that, we need to make sure `AbstractRRset` is the class used
 in other public interfaces.  In particular, Message::addRRset() should
 take (a (shared) pointer to) the base class, rather than some
 intermediate derived class.  In fact, that was the original intention
 of the RRset class hierarchy, but it was broken in a rush of
 completing the year 1 implementation.

 As such, `RRsetPtr` and `ConstRRsetPtr` should be based on
 `AbstractRRset`.

 This should be basically a straightforward task, but many files
 will need to be modified.  So it's better to separate this as a single
 subtask.

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Ticket URL: <http://bind10.isc.org/ticket/1604>
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