BIND 10 #2752: define and implement ZoneDataUpdater::delete()

[BIND 10 Development]

#2752: define and implement ZoneDataUpdater::delete()
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            Reporter:  jinmei        |                        Owner:  muks
                Type:  task          |                       Status:
            Priority:  medium        |  assigned
           Component:  data source   |                    Milestone:
            Keywords:                |  Sprint-20130917
           Sensitive:  0             |                   Resolution:
         Sub-Project:  DNS           |                 CVSS Scoring:
Estimated Difficulty:  6             |              Defect Severity:  N/A
         Total Hours:  0             |  Feature Depending on Ticket:
                                     |  shared memory data source
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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Comment (by muks):

 Replying to [ticket:2752 jinmei]:
 > Delete operation is trickier than add:
 > - when the node becomes empty, and if it's a leaf node in the zone
 >   (i.e., not empty non terminal), we should also delete the
 >   corresponding tree node.
 > - If, as a result of the previous operation, the parent node of the
 >   just-deleted tree node becomes empty, it should also be deleted.
 >   This process continues recursively, toward the zone origin.
 > - But we shouldn't delete the zone origin node (that's the integrity
 >   assumption of our implementation)
 >
 > And, while implementing zone node deletion, we may find the need for
 > additional extensions to `DomainTree`.

 As a pre-cursor before starting implementation, I want to point out that
 #2750 (`DomainTree::remove()`) takes care of these items.

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Ticket URL: <http://bind10.isc.org/ticket/2752#comment:10>
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