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

[BIND 10 Development]

#2834: update ConfigurableClientList::getCachedZoneWriter with new interface
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |  jinmei
            Priority:  medium        |                       Status:
           Component:  Unclassified  |  accepted
            Keywords:                |                    Milestone:
           Sensitive:  0             |  Sprint-20130423
         Sub-Project:  DNS           |                   Resolution:
Estimated Difficulty:  4             |                 CVSS Scoring:
         Total Hours:  0             |              Defect Severity:  N/A
                                     |  Feature Depending on Ticket:
                                     |  shared memory data source
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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Comment (by jinmei):

 trac2834 is ready for review.

 - the first commit (b7a8ae0) is totally unrelated cleanup, but I thought
   it's worth doing and reasonably trivial.  It's probably better to be
   reviewed separately.
 - b7a8ae0..44890bb is the main change of this task.  It's essentially
   an internal refactoring: I moved the code logic of getting
   `LoadAction` from client_list.cc to cache_config.cc, and use the
   corresponding interface of `CacheConfig` for loading and reloading
   zone data.  Test cases needed some adjustments, but I believe they
   are basically straightforward.
 - Last two commit (44890bb..fa304f6).  Now that we don't need to
   maintain the load() related interface in `InMemoryClient`, we can
   simply clean it up with adjusting tests.  The diff is not small but
   should be trivial.  I suggest reviewing this chunk separately.

 Since there should be no externally visible change, I don't think we
 need a changelog entry for this task.

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Ticket URL: <http://bind10.isc.org/ticket/2834#comment:3>
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