BIND 10 #1603: replace name compression with more efficient one

[BIND 10 Development]

#1603: replace name compression with more efficient one
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |  UnAssigned
            Priority:  major         |                       Status:  new
           Component:  libdns++      |                    Milestone:  Year
           Sensitive:  0             |  3 Task Backlog
         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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 This is a subtask for "Replace the name compression logic"
 described in
 https://lists.isc.org/pipermail/bind10-dev/2012-January/002985.html

 It depends on #1602.

 We rewrite the internal implementation of `dns::MessageRenderer`:
 - `MessageRendererImpl` now internally maintains a hash table.
   Its key is `LabelSequence` and the value is corresponding
   compression offset.
 - The general algorithm of writeName is the same as the current
   implementation, but it uses the backend.
 - It should keep reserving resources for some reasonable amount of
   hash entries so that it can be efficient when reused.
 - Since `LabelSequence` needs the underlying name object to be alive,
   we should also keep a copy of the name if the sequence is stored in
   the hash.

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Ticket URL: <http://bind10.isc.org/ticket/1603>
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