BIND 10 #2426: support generic version of rdata::createRdata(text) in generic RDATA

[BIND 10 Development]

#2426: support generic version of rdata::createRdata(text) in generic RDATA
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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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 (I forgot this one.  This may merge in the similar tasks)

 It's the `Generic` rdata class defined in lib/dns/rdata.{h,cc}

 Subtask of #2368, depend on #2374 (and slightly #2377).

 See #2382.

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Ticket URL: <http://bind10.isc.org/ticket/2426>
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