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          |  UnAssigned
            Priority:  medium        |                       Status:  new
           Component:  libdns++      |                    Milestone:  New
           Sensitive:  0             |  Tasks
         Sub-Project:  DNS           |                     Keywords:
Estimated Difficulty:  0             |              Defect Severity:  N/A
         Total Hours:  0             |  Feature Depending on Ticket:
                                     |  loadzone-ng
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 Subtask of #2368.  No dependency.

 In order to support loading RRs of a zone (to memory) coming in any order,
 we need to provide some interface to generate a new `RdataSet` based
 on an existing one with an additional `RRset` (+ maybe RRSIGs).

 If there's any duplicate RR, we need to remove them.  BIND 9 even
 sorts the RDATA (in the DNSSEC RDATA order).  We may or may not do it.

 Note: this will also be needed to support incremental updates after
 DDNS and IXFR.

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Ticket URL: <http://bind10.isc.org/ticket/2440>
BIND 10 Development <http://bind10.isc.org>
BIND 10 Development