BIND 10 #2097: Define and Implement `RdataSet` class

[BIND 10 Development]

#2097: Define and Implement `RdataSet` 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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 As proposed in
 http://bind10.isc.org/wiki/InMemoryZoneDesign#a4.2.RdataSetstructure.

 With the help of separate encoder, this class will be actually pretty
 simple.

 {{{#!cpp
 struct RdataSet {
     // Constructor (again, we may need RRClass too).
     RdataSet(dns::RRType type, dns::RRTTL ttl, uint16_t n_rdata,
              bool with_sig);

     // allocator and deallocator so we can create it from a memory
 segment.
     // allocate() will use RdataEncoder, adding RDATA of rrset and
 sig_rrset,
     // allocate memory, and pass the region to its encode().
     static RdataSet* allocate(MemorySegment& segment,
                               RdataEncoder& encoder,
                               dns::ConstRRsetPtr rrset,
                               dns::ConstRRsetPtr sig_rrset);
     static void deallocate(MemorySegment& segment, RdataSetPtr rdataset);
 }}}

 Depend on #2094 and #2095.

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Ticket URL: <http://bind10.isc.org/ticket/2097>
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