BIND 10 #2087: add LabelSequence::getOffsetData() method

[BIND 10 Development]

#2087: add LabelSequence::getOffsetData() method
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |  UnAssigned
            Priority:  medium        |                       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:
                                     |  scalable inmemory
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 It's the offset version of getData().

 This should be easy, so let's piggy back this: Extend
 MessageRenderer::writeName so it can take LabelSequence We should
 be able to do this so we don't have to construct a name object from
 in-memory encoded data.

 Better to do this after #2086.

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Ticket URL: <http://bind10.isc.org/ticket/2087>
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