BIND 10 #2946: avoid handling DNSServer events after free

[BIND 10 Development]

#2946: avoid handling DNSServer events after free
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                   Reporter:         |                 Owner:
  jinmei                             |                Status:  new
                       Type:         |             Milestone:  Next-Sprint-
  defect                             |  Proposed
                   Priority:         |              Keywords:
  medium                             |             Sensitive:  0
                  Component:         |           Sub-Project:  DNS
  b10-auth                           |  Estimated Difficulty:  0
               CVSS Scoring:         |           Total Hours:  0
            Defect Severity:  N/A    |
Feature Depending on Ticket:         |
        Add Hours to Ticket:  0      |
                  Internal?:  0      |
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 See http://bind10.isc.org/ticket/2942#comment:18

 This task is to give the problem a complete solution.  We'll need to
 do something like including a shared pointer of the server object
 itself, e.g. using `shared_from_this`, in the callback parameter.

 See an example at: http://think-
 async.com/asio/asio-1.4.8/doc/asio/tutorial/tutdaytime3.html#asio.tutorial.tutdaytime3.the_tcp_connection_class

 We'll have to do this for all derived classes of `DNSServer`.

 I also believe we can close #2556 when this task is completed.

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Ticket URL: <http://bind10.isc.org/ticket/2946>
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BIND 10 Development