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

[BIND 10 Development]

#2751: add "subtract RDATA" interface to RdataSet
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                   Reporter:         |                 Owner:
  jinmei                             |                Status:  new
                       Type:  task   |             Milestone:  Next-Sprint-
                   Priority:         |  Proposed
  medium                             |              Keywords:
                  Component:  data   |             Sensitive:  0
  source                             |           Sub-Project:  DNS
               CVSS Scoring:         |  Estimated Difficulty:  0
            Defect Severity:  N/A    |           Total Hours:  0
Feature Depending on Ticket:         |
        Add Hours to Ticket:  0      |
                  Internal?:  0      |
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 We need to extend the `RdataSet` class to create a new `RdataSet`
 object based on an existing one, removing given RDATA and RRSIGs from
 it.  This will be needed to complete the support for incremental
 update to in-memory zone data.

 Consider how to handle corner cases like the given RDATA/RRSIG don't
 exist in the base `RdataSet`.

 This task is probably better to be taken after #2440 is completed.

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