BIND 10 #2441: update in-memory data source so it can load RRs in any order

[BIND 10 Development]

#2441: update in-memory data source so it can load RRs in any order
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  medium        |                    Milestone:  New
           Component:  data source   |  Tasks
           Sensitive:  0             |                     Keywords:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  0             |  Feature Depending on Ticket:
         Total Hours:  0             |  loadzone-ng
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 Subtask of #2368. depend on #2440.

 With the "merge" feature of #2440, we should loosen the current
 restriction, e.g., rejecting adding an `RRset` if the same type of
 set has been already loaded.

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Ticket URL: <http://bind10.isc.org/ticket/2441>
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BIND 10 Development