BIND 10 #621: refactoring in resolver (and auth) for response creation

[BIND 10 Development]

#621: refactoring in resolver (and auth) for response creation
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           Reporter:  jelte          |                      Owner:
               Type:  task           |                     Status:  new
           Priority:  major          |                  Milestone:  feature
          Component:  Unclassified   |  backlog item
          Sensitive:  0              |                   Keywords:
Add Hours to Ticket:  0              |  Estimated Number of Hours:  0
        Total Hours:  0              |                  Billable?:  1
                                     |                  Internal?:  0
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 The DNS callback methods have a answer MessagePtr in which the answer is
 built while we are doing the processing. In some cases, however, this is
 not used (for instance, if we encounter a problem with the query, in which
 case an error response is directly rendered to the buffer we also pass
 along).

 In my initial work for #607, I intended to fix this, refactor
 makeErrorResponse in src/bin/resolver.cc, and make it use the response
 MessagePtr for these error as well (instead of reusing the query_message
 in that case, which would also allow us to make that const). However, this
 resulted in the tests being updated to reflect that (i.e. that response
 message is used instead of query_message).

 These tests are shared with bin/auth, which right now always reuses the
 query message for responses (even though answer messageptr is passed
 around).

 So this ticket is meant to do the initial refactor, and make auth and
 resolver consistent in this regard.

 (big part of the code is already written for the initial work on #607)

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Ticket URL: <https://bind10.isc.org/ticket/621>
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