BIND 10 #2095: Define and implement `RdataEncoder` class

[BIND 10 Development]

#2095: Define and implement `RdataEncoder` class
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  medium        |                    Milestone:  Next-
           Component:  data source   |  Sprint-Proposed
           Sensitive:  0             |                     Keywords:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  0             |  Feature Depending on Ticket:
         Total Hours:  0             |  scalable inmemory
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 The name and the class spec are derived from my experiments, but it
 doesn't have to be that way.  Think about the best design and
 implement it.  In my experimental version it would look like:

 {{{#!cpp
 class RdataEncoder : boost::noncopyable {
 public:
     RdataEncoder(); // can be default-constructed

     // Internally "add" the given RDATA.
     void addRdata(const dns::rdata::Rdata& rdata);

     // Internally "add" RRSIG for the RDATA.  It's supposed to be of
 RRSIG.
     void addSigRdata(const dns::rdata::Rdata& rdata);

     // After a sequence of calls to addRdata()/addSigRdata(), prepare
     // internally state for actual encoding.  call to addXXX after
     // this point will be invalid.
     void construct(dns::RRType type);

     // Return the size of storage for the entire data in the encoder.
     // only valid after construct().  The caller will allocate necessary
     // storage using it.
     size_t getStorageLength() const;

     // encode the internal data  starting from the given pointer.
     // the caller should have allocated sufficient space.
     size_t encode(void* data, size_t data_len) const;

     // Reset internal state for reuse.  After this call, the caller can
     // restart encoding from addXXX.
     void clear();
 }}}

 This class will internally use something similar to things internally
 used in libdns++'s RdataFields (specialized renderer).

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Ticket URL: <http://bind10.isc.org/ticket/2095>
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