BIND 10 #2166: update RRset::toWire() so it will render RRSIGs, too

[BIND 10 Development]

#2166: update RRset::toWire() so it will render RRSIGs, too
-------------------------------------+-------------------------------------
            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:
                                     |  scalable inmemory
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 This is the final building block of #2163.

 We update the abstract interface of `RRset::toWire()` so that if it's
 associated with RRSIG it also renders the RRSIGs.  Then update the
 actual implementation accordingly.  Finally, adjust test cases that
 assume the old behavior.  Also re-enable any disabled tests in #2165.

 Depends on #2165.

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Ticket URL: <http://bind10.isc.org/ticket/2166>
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