BIND 10 #2652: Implement `getUpdater()` in `InMemoryClient`

[BIND 10 Development]

#2652: Implement `getUpdater()` in `InMemoryClient`
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                   Reporter:  muks   |                 Owner:
                       Type:         |                Status:  new
  enhancement                        |             Milestone:  New Tasks
                   Priority:         |              Keywords:
  medium                             |             Sensitive:  0
                  Component:  data   |           Sub-Project:  DNS
  source                             |  Estimated Difficulty:  0
               CVSS Scoring:         |           Total Hours:  0
            Defect Severity:  N/A    |
Feature Depending on Ticket:         |
        Add Hours to Ticket:  0      |
                  Internal?:  0      |
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 This ticket is an offshoot of #2499.

 Implement a `ZoneUpdater` and make `InMemoryClient::getUpdater()` return
 it. Also use this updater for zone validation and remove the call to
 `dns::checkZone()` in `loadZoneDataInternal()`.

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Ticket URL: <http://bind10.isc.org/ticket/2652>
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