BIND 10 #1060: introduce the abstract DataSourceClient and ZoneHandle classes

[BIND 10 Development]

#1060: introduce the abstract DataSourceClient and ZoneHandle classes
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  major         |                    Milestone:  Next-
           Component:  data source   |  Sprint-Proposed
           Sensitive:  0             |                     Keywords:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  0             |  Feature Depending on Ticket:
         Total Hours:  0             |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 This is a first step of data source refactoring.

 - DataSourceClient: abstracted and bit renamed from MemoryDataSource.
   MemoryDataSource will become a derived class of it, and would be
   renamed to MemoryClient (or InMemoryClient or
   MemoryDataSourceClient, depending on our naming preference)
 - ZoneHandle: (ditto from "Zone/MemoryZone").
 - update the auth server's Query class so that it will use the
   abstract version and be agnostic about the data source type.

 The amount of change will be a bit big but it should be essentially a
 trivial task.

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Ticket URL: <http://bind10.isc.org/ticket/1060>
BIND 10 Development <http://bind10.isc.org>
BIND 10 Development