BIND 10 #2205: introduce a "data source configurator" thread in auth

[BIND 10 Development]

#2205: introduce a "data source configurator" thread in auth
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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:
         Total Hours:  0             |  background zone loading
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 A subtask of #2201.

 First, fully consider how to test these without creating threads.

 In this task, we let auth spawns a separate thread with a
 synchronization mechanism with the main auth thread.  The main and
 configurator thread work as the producer and the consumer: the main
 thread (producer) passes general form of command in the form of
 `ConstElementPtr` and the configurator thread (consumer) accepts each
 command and execute it.

 Choose an appropriate synchronization mechanism for the concept and
 implement it.  One way is to let both thread share
 `vector<ConstElementPtr>` with a conditional variable to get access to
 it.

 Currently, we only support "shutdown" command.  On auth's shutdown,
 the main thread sends this command to configurator, and waits for it
 to day by join().  The configurator simply exits if it gets this
 command.

 Same open question as in #2202 applies.  We use the consistent policy
 on this.

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Ticket URL: <http://bind10.isc.org/ticket/2205>
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