BIND 10 #2834: update ConfigurableClientList::getCachedZoneWriter with new interface

[BIND 10 Development]

#2834: update ConfigurableClientList::getCachedZoneWriter with new interface
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                   Reporter:         |                 Owner:
  jinmei                             |                Status:  new
                       Type:  task   |             Milestone:  Next-Sprint-
                   Priority:         |  Proposed
  medium                             |              Keywords:
                  Component:         |             Sensitive:  0
  Unclassified                       |           Sub-Project:  DNS
               CVSS Scoring:         |  Estimated Difficulty:  0
            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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 subtask of #2830, and continuation of #2833.

 Using the revised feature of #2833, we should be able to simplify
 this method, as zone table segment now knows more details of how to
 load the zone (e.g., the file name of the zone).

 Also, at this point, we should be able to remove file names from
 `MemoryClient`.  This task should do this cleanup too.

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Ticket URL: <http://bind10.isc.org/ticket/2834>
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BIND 10 Development