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

[BIND 10 Development]

#2752: define and implement ZoneDataUpdater::delete()
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                   Reporter:         |                 Owner:
  jinmei                             |                Status:  new
                       Type:  task   |             Milestone:  Next-Sprint-
                   Priority:         |  Proposed
  medium                             |              Keywords:
                  Component:  data   |             Sensitive:  0
  source                             |           Sub-Project:  DNS
               CVSS Scoring:         |  Estimated Difficulty:  0
            Defect Severity:  N/A    |           Total Hours:  0
Feature Depending on Ticket:         |
        Add Hours to Ticket:  0      |
                  Internal?:  0      |
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 It's the delete operation version of `ZoneDataUpdater::add()`, takes
 normal and RRSIG rrsets to be deleted from the zone.

 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`.

 All of these things may require breaking this feature into several sub
 tasks, but at this moment I'm not sure.

 It at least depends on #2750 and #2751.

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