BIND 10 #1793: update b10-auth command handler to reload in-memory/sqlite3

[BIND 10 Development]

#1793: update b10-auth command handler to reload 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 update auth/command.cc:LoadZoneCommand so that it identifies
 whether the zone is generated from a normal file or sqlite3 file (a
 simplest way would be to exploit InMemoryZoneFinder::getFileName().
 There may be a better way), and in the latter case reload the zone via
 a `ZoneIterator` for the underlying SQLite3 data source (use #1791).

 It depends on #1791.

 See also #1792.  The command handler needs to instantiate a
 DataSourceClient for the corresponding SQLite3 data source.  Right now
 I'm not sure how easy/difficult it is.  While this ticket could be
 independent from #1792, it's probably better to unify this part.
 So there should be some kind of coordination.

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Ticket URL: <http://bind10.isc.org/ticket/1793>
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BIND 10 Development