BIND 10 #2751: add "subtract RDATA" interface to RdataSet

[BIND 10 Development]

#2751: add "subtract RDATA" interface to RdataSet
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |  vorner
            Priority:  medium        |                       Status:
           Component:  data source   |  reviewing
            Keywords:                |                    Milestone:
           Sensitive:  0             |  Sprint-20130917
         Sub-Project:  DNS           |                   Resolution:
Estimated Difficulty:  5             |                 CVSS Scoring:
         Total Hours:  0             |              Defect Severity:  N/A
                                     |  Feature Depending on Ticket:
                                     |  shared memory data source
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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Comment (by muks):

 Replying to [comment:9 muks]:
 > * We check that the RRsets are of the same class, same
 >   RRtype, etc. Where are the names checked?

 Actually in hindsight, I guess it is not possible to check the names as
 the RdataSets are hanging off tree nodes and don't know their names
 without being able to access the node in question (which they don't have).

 From the testcases in `RdataSetTest.subtract`, it did the subtract and
 passed the test when the RRsets to be subtracted had a different name
 (`www.example.org`). But in real usecase, this doesn't matter.

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Ticket URL: <http://bind10.isc.org/ticket/2751#comment:10>
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