BIND 10 #552: get the "closest previous node" in RBTree find()

[BIND 10 Development]

#552: get the "closest previous node" in RBTree find()
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           Reporter:  jinmei         |                      Owner:
               Type:  enhancement    |                     Status:  new
           Priority:  major          |                  Milestone:  A-Team-
          Component:  data source    |  Task-Backlog
          Sensitive:  0              |                   Keywords:
Add Hours to Ticket:  0              |  Estimated Number of Hours:  0
        Total Hours:  0              |                  Billable?:  1
                                     |                  Internal?:  0
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 This is a general extension to the RBTree class, but is mainly intended
 to server as a subtask of wildcard handling for in-memory data source.
 See the analysis ticket (#506) for the big picture.

 The feature to be implemented is a new extension to RBTree::find(),
 which remembers th "previous existing node" for the query name if an
 exact match isn't found. This is necessary for a minor case of
 wildcard matching, but will also be necessary for DNSSEC later.

 This task can be handled separately from other wildcard related tasks,
 but it will use the framework of #517, so it should be done after
 #517.

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Ticket URL: <https://bind10.isc.org/ticket/552>
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