BIND 10 #2440: add support for "merging" RRsets into memory::RdataSet

[BIND 10 Development]

#2440: add support for "merging" RRsets into memory::RdataSet
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |  jinmei
            Priority:  medium        |                       Status:
           Component:  data source   |  accepted
            Keywords:                |                    Milestone:
           Sensitive:  0             |  Sprint-20130219
         Sub-Project:  DNS           |                   Resolution:
Estimated Difficulty:  6             |                 CVSS Scoring:
         Total Hours:  0             |              Defect Severity:  N/A
                                     |  Feature Depending on Ticket:
                                     |  loadzone-ng
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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Comment (by jinmei):

 trac2440 is ready for review.

 The diff is a bit large, but they can be clearly staged, so I believe
 it's not that difficult to review it if done stage by stage.
 Guideline:

 - master...016d4a4 This part enhances `RdataEncoder` so it will unify
   any duplicate data.
 - 016d4a4..4ef474e On top of the first part, it enhances
   `RdataEncoder` so it takes previously encoded data and merge
   additional data (in a separate storage).
 - 4ef474e..759ad9e Finally, update the `RdataSet` class for the main
   goal of this task.
 - 896dd15 is an additional documentation update.

 With this task completed, there'll be one user-visible change:
 duplicate RRs will be suppressed in in-memory.  But I'd defer
 changelog update including this to #2441.

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Ticket URL: <http://bind10.isc.org/ticket/2440#comment:16>
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