BIND 10 #1747: refactor auth::Query so it's reusable

[BIND 10 Development]

#1747: refactor auth::Query so it's reusable
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  critical      |                    Milestone:  Next-
           Component:  b10-auth      |  Sprint-Proposed
           Sensitive:  0             |                     Keywords:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  0             |  Feature Depending on Ticket:  auth
         Total Hours:  0             |  performance
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 The auth::Query class is now (going to) use internal resource
 to hold RRsets with #1607.  In #1607 I used a local
 vector<ConstRRsetPtr> object, but it's not efficient to allocate it
 for every query.

 I propose making auth::Query reusable and reusing the internal
 resource within the class.  Then the auth server creates a Query
 object on its initialization and keeps using it.  Make sure the
 internal resource is always cleared for every query, especially when
 some query processing results in an exception.  It would also be
 better to ensure the capacity of the vector(s) won't exceed some
 limit after cleanup.

 This would be a relatively easy task, so I'd also make one more
 refactoring in this ticket: using separate vectors for the answer and
 authority sections and storing RRsets for these sections in the middle
 of the query processing.  Then we can conslidate the response message
 construction at the end of the method:

 {{{#!c++
     for_each(answers.begin(), answers.end(),
              RRsetInserter(response_, Message::SECTION_ANSWER, dnssec_));
     for_each(authorities.begin(), authorities.end(),
              RRsetInserter(response_, Message::SECTION_AUTHORITY,
 dnssec_));
     for_each(additionals.begin(), additionals.end(),
              RRsetInserter(response_, Message::SECTION_ADDITIONAL,
                            dnssec_));
 ]}}

 which would be clearer, and will make #1688 easier to handle.

 It's not direct dependency, but it's probably better to start this
 task after #1607.

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Ticket URL: <http://bind10.isc.org/ticket/1747>
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