BIND 10 #2219: switch to memory-efficient version of in memory data source

[BIND 10 Development]

#2219: switch to memory-efficient version of in memory data source
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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:  0             |  Feature Depending on Ticket:
         Total Hours:  0             |  scalable inmemory
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 As already noted in #2101 we'll need to migrate to the new version
 of the in-memory data source once all other tasks in #2101 are
 completed.

 It will be to update the place where we instantiate
 `memory::InMemoryClient`
 instead of the current bare `InMemoryClient` (maybe adjusting
 constructor parameters), confirm all tests pass (hopefully most will,
 but there'll probably be some regressions, so fix them), then
 remove the old implementation.

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Ticket URL: <http://bind10.isc.org/ticket/2219>
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BIND 10 Development