BIND 10 #1605: introduce special RRset for in memory data source

[BIND 10 Development]

#1605: introduce special RRset for in memory data source
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  major         |                    Milestone:  Year
           Component:  data source   |  3 Task Backlog
           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 is a subtask for "Pre-establish shortcuts for additional section
 processing" described in
 https://lists.isc.org/pipermail/bind10-dev/2012-January/002985.html
 and a followup task of #1604.

 We define a dedicated derived class of `AbstractRRset` used within the
 in memory data source implementation (tentatively named `RBNodeRRset`
 in my experimental version, but can be different).  It internally
 holds the basic `RRset` (in the form of `ConstRRsetPtr`) and forward
 all public methods to the composed `RRset`.

 On loading the zone, the in memory data source implementation
 constructs an `RBNodeRRset` for each given `ConstRRsetPtr` and
 maintains it as the data source data.  It returns the special RRset as
 the return value of the find() method.

 It will eventually contain the trick of enabling additional section
 shortcut, but that will go to a separate task.

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Ticket URL: <http://bind10.isc.org/ticket/1605>
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