BIND 10 #1792: update b10-auth config handler to support in-memory/sqlite3

[BIND 10 Development]

#1792: update b10-auth config handler to support in-memory/sqlite3
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  medium        |                    Milestone:  Next-
           Component:  b10-auth      |  Sprint-Proposed
           Sensitive:  0             |                     Keywords:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  0             |  Feature Depending on Ticket:  xfr
         Total Hours:  0             |  for in-memory
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 This is a subtask of #1787.

 We extend auth_config.cc:MemoryDatasourceConfig so that if the
 "filetype" is "sqlite3" (see #1788), it creates a `ZoneIterator` for
 the underlying SQLite3 data source and uses the iterator version of
 InMemoryZoneFinder::load() (see #1791) to load the zone.

 Note that this means the config handler needs to instantiate a
 `DataSourceClient` for the corresponding SQLite3 data source.
 Right now I'm not sure how easy/difficult it is.

 This task depends on #1788 and #1791.

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Ticket URL: <http://bind10.isc.org/ticket/1792>
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BIND 10 Development