BIND 10 #2497: introduce wrapper version of "from lexer" rdata factory

[BIND 10 Development]

#2497: introduce wrapper version of "from lexer" rdata factory
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |  UnAssigned
            Priority:  medium        |                       Status:  new
           Component:  libdns++      |                    Milestone:  New
           Sensitive:  0             |  Tasks
         Sub-Project:  DNS           |                     Keywords:
Estimated Difficulty:  0             |              Defect Severity:  N/A
         Total Hours:  0             |  Feature Depending on Ticket:
                                     |  loadzone-ng
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 This task is to introduce an intermediate wrapper so that we can use
 the "from master lexer" Rdata factory transparently, until we complete
 supporting all currently defined Rdata's.  The basic idea is to
 forward the work to the existing "from string" factory by default, and
 use the dedicated versions as we implement those RR types of Rdata.

 Specifically:

 - introduce `AbstractRdataFactory::create(lexer)`:
 {{{#!cpp
 class AbstractRdataFactory {
     virtual RdataPtr create(MasterLexer& lexer, const Name* origin,
                             MasterLoader::Option options,
                             MasterLoader::Callbacks& callbacks) const;
 };
 }}}
   and define the default version of it (unlike other create() methods
   we need the default version for the moment).  It will use the lexer
   to get all token until EOL or EOF as strings, concatenate them
   separated by a space into a single std::string object, and then
   call this version of create():
 {{{#!cpp
     virtual RdataPtr create(const std::string& rdata_str) const = 0;
 }}}
 - update rrparamregistry-placeholder.cc as follows:
   - rename current `RdataFactory` to `OldRdataFactory`.  for the "from
     lexer" create() method, it will use the default version.
   - introduce a new `RdataFactory`, which has the "from lexer" create
     method:
 {{{#!cpp
     virtual RdataPtr create(MasterLexer& lexer, const Name* origin,
                             MasterLoader::Option options,
                             MasterLoader::Callbacks& callbacks) const
     {
         return (RdataPtr(new T(dynamic_cast<const T&>(lexer, origin,
 options,
                                                       callbacks))));
     }
 }}}
 - update gen-rdatacode.py:generate_rrparam() so that, except for
   hardcoded exceptions, use `OldRdataFactory` instead of `RdataFactory`.
   we update the exception list as we implement RDATAs

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Ticket URL: <http://bind10.isc.org/ticket/2497>
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