BIND 10 #2212: support "load zone" command in the configurator thread

[BIND 10 Development]

#2212: support "load zone" command in the configurator thread
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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, depend on #2202, #2203, #2205, #2209

 This task extends the newly introduced "data source configurator"
 thread (#2205) to support background zone reloading.

 It will be based on the current content of LoadZoneCommand::exec(),
 but works as follows:
 - first call `ConfigurableClientList::getCacheZoneUpdater()`
   introduced in #2209
 - then call updater->load()
 - then acquire the lock introduced in #2202
 - then call updater->install()
 - then release the lock
 - then call updater->cleanup().

 Note that updater operations can throw.

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Ticket URL: <http://bind10.isc.org/ticket/2212>
BIND 10 Development <http://bind10.isc.org>
BIND 10 Development