BIND 10 #1602: Introduce dns::LabelSequence class

[BIND 10 Development]

#1602: Introduce dns::LabelSequence class
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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

 (I was a bit incorrect in the email.  This one slightly depended on
 the "more advanced" LabelSequence class.  We could implement this
 optimization without it, but I'm afraid it's less efficient).

 The `LabelSequence` class is a lightweight accessor to a name object.
 It internally maintains wire-format name data and offsets to each
 label, and allow the caller to "split" it by simply switching some
 internal pointers.

 It (for the purpose of this task) offers the following methods:
 - equals(other, case_sensitive): check equality with another
   `LabelSequence`.  can be case sensitive (off by default)
 - split(n): if n > 0, strip the first n labels of the sequence.  if n
   < 0, strip the last n labels of the sequence.  The result can be
   "non absolute" sequence, i.e, a sequence not terminated by a null
   label (this is different from Name)
 - getLabelCount(): return the number of labels
 - getData(): similar to string::c_str().

 We also add a generator from a Name object: Name::toLabelSequence(),
 which will create and return a `LabelSequence` corresponding to the
 name (so the underlying name object must be retained throughout the
 lifetime of the label sequence).

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Ticket URL: <http://bind10.isc.org/ticket/1602>
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