BIND 10 #2100: Update ZoneTable class

[BIND 10 Development]

#2100: Update ZoneTable class
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                   Reporter:         |                 Owner:  jinmei
  jinmei                             |                Status:  accepted
                       Type:  task   |             Milestone:
                   Priority:         |  Sprint-20120821
  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):

 trac2100 is ready for review.

 In the first commit (57de755) I copied the old implementation.  This
 commit can be ignored for review.  But it may be easier to read the
 entire implementation rather than taking the diff from the initial
 commit.  It's not that big.

 I've added a mostly empty implementation of `ZoneData` (without direct
 for this class at the moment).  At this moment it's mostly a
 placeholder of the zone table.

 On looking into the details, I decided to change the return type of
 findZone(); instead of creating a zone finder here, it now just
 returns the underlying `ZoneData` and let the caller to use it (e.g.
 for creating a finder containing the data).  It seemed cleaner and
 will make it easier to use the zone table for different purposes (like
 creating an updater).

 The last two commits are not directly related to the subject of this
 branch, but I thought they're necessary and should be simple enough.
 See the commit logs.  If these happen to be controversial it's okay
 for me to exclude them in this ticket.

-- 
Ticket URL: <http://bind10.isc.org/ticket/2100#comment:6>
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