BIND 10 #1697: extend MessageRenderer so we can replace the output buffer

[BIND 10 Development]

#1697: extend MessageRenderer so we can replace the output buffer
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |  UnAssigned
            Priority:  major         |                       Status:  new
           Component:  libdns++      |                    Milestone:  Next-
           Sensitive:  0             |  Sprint-Proposed
         Sub-Project:  DNS           |                     Keywords:
Estimated Difficulty:  0             |              Defect Severity:  N/A
         Total Hours:  0             |  Feature Depending on Ticket:  auth
                                     |  performance
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 Right now the buffer needs to be passed to a `MessageRenderer` on
 construction and stay there all the time.  But we should probably
 need to make it replaceable so b10-auth keep an instance of renderer
 and reuse it, while "attaching" a given output buffer passed from
 the asiodns module.  In future, we may even introduce
 datasource-customized renderer that optimizes compression logic
 exploiting the knowledge of the data source internal.  This feature
 will also be important if and when we do that.

 I think this will be part of the "optimized UDPServer" feature.

 It will include some design work to make reasonable API extension, so
 it may not be very easy.

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Ticket URL: <http://bind10.isc.org/ticket/1697>
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BIND 10 Development