BIND 10 #2850: Define and implement ZoneTableSegmentMapped

[BIND 10 Development]

#2850: Define and implement ZoneTableSegmentMapped
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            Reporter:  jinmei        |                        Owner:  muks
                Type:  task          |                       Status:
            Priority:  medium        |  reviewing
           Component:  data source   |                    Milestone:
            Keywords:                |  Sprint-20130611
           Sensitive:  0             |                   Resolution:
         Sub-Project:  DNS           |                 CVSS Scoring:
Estimated Difficulty:  5             |              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):

 Hi Jinmei

 These patches were fine. I have added them to the branch.

 `trac2850_4` was merged to `master` in commit
 `587f61464634214c7cdbbb204652eb63b11eea3b`. This is not a straightforward
 merge as the `ZoneWriter` in master had moved on since then. Please review
 this merge commit, after which I'll close this bug.

 Testing for the `impl_->data_holder_` `scoped_ptr` as a `bool` generated
 `true` for a `NULL` value, so I tested the result of `.get()`.

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Ticket URL: <http://bind10.isc.org/ticket/2850#comment:37>
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