BIND 10 #2292: eliminate const_cast from domaintree.h

[BIND 10 Development]

#2292: eliminate const_cast from domaintree.h
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            Reporter:  jinmei        |                        Owner:
                Type:  defect        |                       Status:  new
            Priority:  medium        |                    Milestone:  Next-
           Component:  data source   |  Sprint-Proposed
           Sensitive:  0             |                     Keywords:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  0             |  Feature Depending on Ticket:
         Total Hours:  0             |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 I don't know whether this was discussed in code review, but I don't
 think we should have allowed this:

 {{{#!cpp
 typename DomainTree<T>::Result
 DomainTree<T>::find(const isc::dns::LabelSequence& target_labels_orig,
                     DomainTreeNode<T>** target,
                     DomainTreeNodeChain<T>& node_path,
                     bool (*callback)(const DomainTreeNode<T>&, CBARG),
                     CBARG callback_arg) const
 {
 [...]
     if (!node_path.isEmpty()) {
         // Get the top node in the node chain
         node = const_cast<DomainTreeNode<T>*>(node_path.top());
 }}}

 If we do const_cast, there should basically be something wrong.  In
 the previous version of the in-memory data source read-only and modify
 cases are not cleanly separated and there could be a reason why we
 cannot easily make things in the find() (read-only) path const.  The
 new version should be clean enough, and we should make sure we don't
 need beasts const_cast.

 From a quick check it seems we need to introduce some non trivial
 adjustments, but I believe it should be possible and won't require
 fundamental changes to the public interfaces.

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Ticket URL: <http://bind10.isc.org/ticket/2292>
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