BIND 10 #1805: implement getClosestNSEC() within InMemoryZoneFinder

[BIND 10 Development]

#1805: implement getClosestNSEC() within InMemoryZoneFinder
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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:  in-
         Total Hours:  0             |  memory NSEC
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 This is a new (tentatively named) internal helper function (or method)
 of the in-memory finder.  It repeatedly calls
 RBTreeNodeChain::getPreviousNode() (see #1803) for the node chain used
 in find(), until it finds a node returned by getPreviousNode() that
 has NSEC (note there are cases where the node returned by
 getPreviousNode() doesn't have NSEC in the case of glue RRs).  Then it
 returns the found NSEC RRset.

 It depends at least on #1803.  Eventually it will need #1804 but can
 be started as soon as #1803 is completed (or even before that, as soon
 as the interface of getPreviousNode() is fixed if we want maximum
 concurrency).

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Ticket URL: <http://bind10.isc.org/ticket/1805>
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