BIND 10 #2107: redefine in-memory zone data

[BIND 10 Development]

#2107: redefine in-memory zone data
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                   Reporter:         |                 Owner:  jinmei
  jinmei                             |                Status:  accepted
                       Type:  task   |             Milestone:
                   Priority:         |  Sprint-20120904
  medium                             |            Resolution:
                  Component:  data   |             Sensitive:  0
  source                             |           Sub-Project:  DNS
                   Keywords:         |  Estimated Difficulty:  4
            Defect Severity:  N/A    |           Total Hours:  0
Feature Depending on Ticket:         |
  scalable inmemory                  |
        Add Hours to Ticket:  0      |
                  Internal?:  0      |
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Comment (by jinmei):

 trac2107 is ready for review.

 I believe the design and implementation are basically straightforward
 in the end, but I needed to do some small setups, and so the diff
 is a bit large.  I hope the following clarification will help:

 - First commit (6ca7b0d) is a merge for other branch to use it in this
   task.  It should be ignored.
 - 7923aa4 is an independent change to `DomainTree`.  It may not be
   needed for this task, but we'll eventually need it anyway.
 - I needed to make one other big set of changes to `DomainTree` at
   6679f42.  See the commit log for details.  The size of changes is
   pretty big, but as commented most of them should be straightforward
   adjustment.
 - 3128629 is an unrelated extension for easier tests.  I could have
   completed this task without it, but I hope it's useful for others
   and the diff should be reasonably small and trivial.
 - two new files segment_object_holder.h and memory_segment_test.h were
   borrowed from #2100, and I extracted them to separate files as I
   expect we'll share them with #2100 at the point of merge.  These can
   be excluded from review because they essentially belong to #2100.
 - Other than these, the main changes are newly introduced zone_data.h,
   zone_data.cc and its tests.  They are fully documented in .h, so
   please read the documentation.  But I believe the implementation is
   basically pretty straightforward.

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Ticket URL: <http://bind10.isc.org/ticket/2107#comment:6>
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