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

[BIND 10 Development]

#1599: introduce "asiodns::UDPSyncServer"
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                   Reporter:         |                 Owner:  UnAssigned
  jinmei                             |                Status:  reviewing
                       Type:  task   |             Milestone:
                   Priority:  major  |  Sprint-20120306
                  Component:         |            Resolution:
  Unclassified                       |             Sensitive:  0
                   Keywords:         |           Sub-Project:  DNS
            Defect Severity:  N/A    |  Estimated Difficulty:  7
Feature Depending on Ticket:  auth   |           Total Hours:  0
  performance                        |
        Add Hours to Ticket:  0      |
                  Internal?:  0      |
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Changes (by vorner):

 * owner:  vorner => UnAssigned
 * status:  accepted => reviewing


Comment:

 Hello

 It should be ready for review.

 I do not change the interface, therefore I do check the checkin callback.
 I think we want to eliminate it globally, because we do check our
 configuration updates somewhere else. But that would be a different
 ticket. It, however, rejects some functions which are virtuals (if it
 didn't reimplement them and used the inherited ones and someone called it,
 we would get an infinite loop, this insures nobody wants to use the sync
 server when doing something asynchornous).

 I use the typed tests to run the whole testcase once for ordinary UDP
 server and once for the synchronous one (the synchronous one got two more
 tests, to see if there are the correct exceptions).

 While implementing it, I removed some functions that are not used
 anywhere, so I didn't have to implement them. The actual implementation
 should be easy to follow (but it is better to read the final
 sync_udp_server.cc than to follow the whole history of the file, because I
 started by copying the udp server to check the tests are duplicated
 correctly).

 I think the best way to move on with #1600 is to add a boolean variable to
 the DNSService class and create either UDPServer or SyncUDPServer,
 depending on the variable.

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Ticket URL: <http://bind10.isc.org/ticket/1599#comment:6>
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