BIND 10 #2309: define ZoneFinder::findAtOrigin()

[BIND 10 Development]

#2309: define ZoneFinder::findAtOrigin()
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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             |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 While working on #2284 I realized the proposed
 `ZoneFinder::Context::getAtOrigin()` doesn't work for the originally
 intended purpose.  `Query::addAuthAdditional()` needs another
 finder context so it can retrieve additional records, so what I
 actually needed an extension to `ZoneFinder` itself:

 {{{#!cpp
 // Work like find(), but search at the zone's origin.
 boost::shared_ptr<ZoneFinder::Context>
 ZoneFinder::findAtOrigin(const isc::dns::RRType& type,
                          const FindOptions options);
 }}}

 Like the `Context` methods, I propose providing the default version
 that works for any data source (that would be a straightforward
 wrapper of `find(finder.getOrigin(), ..)`.

 In this ticket, we define the general interface, implement the default
 version, and actually use it in auth `Query` class.

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Ticket URL: <http://bind10.isc.org/ticket/2309>
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