BIND 10 #1749: implement RBNodeRRset::isSameKind()

[BIND 10 Development]

#1749: implement RBNodeRRset::isSameKind()
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  critical      |                    Milestone:  Next-
           Component:  data source   |  Sprint-Proposed
           Sensitive:  0             |                     Keywords:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  0             |  Feature Depending on Ticket:  auth
         Total Hours:  0             |  performance
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 This task implements a specialized (optimized) version of #1748.

 What I'd envision is:

 - if `other` is of the RBNodeRRset class, too (we need RTTI here),
   (basically) just check the equality of the underlying pointer.
   Base on how RBNodeRRset instances are given to apps this simple and
   efficient check should be sufficient.
 - if `other` is of a different derived class (it's possible if it's
   wildcard expansion or DNAME substitution), use the naive comparison.

 It depends on #1748.  No other dependency.

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Ticket URL: <http://bind10.isc.org/ticket/1749>
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BIND 10 Development