BIND 10 #3326: Modify ZoneIterator interface to enforce getSOA() requirements

[BIND 10 Development]

#3326: Modify ZoneIterator interface to enforce getSOA() requirements
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                   Reporter:  muks   |                 Owner:
                       Type:         |                Status:  new
  enhancement                        |             Milestone:  New Tasks
                   Priority:         |              Keywords:
  medium                             |             Sensitive:  0
                  Component:  data   |           Sub-Project:  DNS
  source                             |  Estimated Difficulty:  0
               CVSS Scoring:         |           Total Hours:  0
            Defect Severity:  N/A    |
Feature Depending on Ticket:         |
        Add Hours to Ticket:  0      |
                  Internal?:  0      |
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 `ZoneIterator::getSOA()`'s API doc requires us to cache the SOA RRset
 during iterator construction. This is so the SOA does not change after
 iteration starts even if there are changes to the underlying data source.
 This is currently suggested in API doc and not enforced, and our data
 sources implement it. We should consider modifying the `ZoneIterator`
 interface such that `getSOA()` is no longer virtual and the SOA RRset is
 cached by the parent `ZoneIterator` itself during construction.

 The API for this would be a little tricky as `ZoneIterator` is constructed
 before derived classes, so we may either use a `ZoneIteratorImpl` that
 `ZoneIterator` wraps around, or get the derived constructor to call a
 protected `setSOA()` method.

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Ticket URL: <https://bind10.isc.org/ticket/3326>
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