BIND 10 #1599: introduce "asiodns::UDPSyncServer"

[BIND 10 Development]

#1599: introduce "asiodns::UDPSyncServer"
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  major         |                    Milestone:  Year
           Component:  Unclassified  |  3 Task Backlog
           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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 This is a subtask for "Eliminate the overhead of ASIO wrapper"
 described in
 https://lists.isc.org/pipermail/bind10-dev/2012-January/002985.html

 It's a lightweight version of asiodns::UDPServer:
  - reusing the socket, message, buffer and endpoint for all requests.
  - not use the "coroutine" trick
  - the ASIO callback for async_read simply calls lookup_callback_, and
    immediately send the result using send_to (this doesn't have to
    async)
  - it also doesn't care about "checkin_callback_".  commands/config
    changes are managed separately in ASIO, so it doesn't have to be
    integrated in to this class.  (actually I suspect the original
    UDPServer doesn't have to handle it within it either, but that's a
    separate issue)

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Ticket URL: <http://bind10.isc.org/ticket/1599>
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