BIND 10 #2098: Define and Implement `TreeNodeRRset` class

[BIND 10 Development]

#2098: Define and Implement `TreeNodeRRset` class
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  medium        |                    Milestone:  Next-
           Component:  data source   |  Sprint-Proposed
           Sensitive:  0             |                     Keywords:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  0             |  Feature Depending on Ticket:
         Total Hours:  0             |  scalable inmemory
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 (Since some people seem to react to the term "RBTree" knee-jerk, this
 is given a more generic name.  For the same reason we might want to
 make it a templated class).

 This is a revised version of `RBNodeRRset`.  It will take `RBNode` (or
 "`TreeNode`") and `RdataSet` and hold pointers to them.  All of its
 public interfaces are implemented through the underlying tree node and
 RdataSet.  In the initial version, we could omit many of them "not
 implemented": all setXXX can be skipped.  Even getRdataIterator() can
 be skipped.  Some can be inefficient for now (e.g. getName()).

 The only important part is the toWire() method.  It will use the
 encoded version of iterator, and render names using `LabelSequence`.

 Depends on #2097 and (at least) #2093.

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Ticket URL: <http://bind10.isc.org/ticket/2098>
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