BIND 10 #2160: b10-auth should clear Message at the end of request handling

[BIND 10 Development]

#2160: b10-auth should clear Message at the end of request handling
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            Reporter:  jinmei        |                        Owner:
                Type:  defect        |                       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             |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 In the current code, the `Message` object keeps all `RRsets` stored
 in the response, and they are cleared at the beginning of next request
 handling.

 With the revised version of in-memory data source this is not good
 because the `RRsets` would have a reference to internal data structures
 of the data source (tree node, `RdataSet` object, etc), and if the
 in-memory image is replaced due to like reload between the two
 requests, the `Message` clear operation will lead to some undesirable
 behavior.

 We should revise the code so that the `Message` is always cleared
 at the end of request handling (note: it shouldn't affect sending the
 response data once rendering is completed).  We'd probably extend
 the existing `RendererHolder` helper class to support both the
 renderer and the message.

 When this is done, I believe we can remove the workaround in
 ~AuthSrvText().

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Ticket URL: <http://bind10.isc.org/ticket/2160>
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BIND 10 Development