BIND 10 #2099: Update InMemoryDataSource

[BIND 10 Development]

#2099: Update InMemoryDataSource
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  medium        |                    Milestone:  Next-
           Component:  data source   |  Sprint-Proposed
           Sensitive:  0             |                     Keywords:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  meta          |  Feature Depending on Ticket:
         Total Hours:  0             |  scalable inmemory
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 This won't be a super heavy task as it might look.  In some sense it
 could be quite close to straightforward refactoring.  But we'll
 definitely need multiple sub tickets.  Right now (due to lack of
 time), I'll just list needed items:

 - Some clarification on internal helper classes to improve readability
   and maintainability
 - Update the load() part so it will use the new encoding for the RDATA.
   Note that we don't introduce name shortcut for now, so it will be
   even simplified.
 - Update find() and related method so it will use `RdataSet` instead
   of the full `RRset`.  It will need to construct
 - Additional section handling.  While there's no shortcut yet, it's
   probably better to implement it in a separate context class.

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Ticket URL: <http://bind10.isc.org/ticket/2099>
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BIND 10 Development