BIND 10 #1398: refactor _reply_xfrout_query

[BIND 10 Development]

#1398: refactor _reply_xfrout_query
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  critical      |                    Milestone:  Next-
           Component:  xfrout        |  Sprint-Proposed
           Sensitive:  0             |                     Keywords:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  0             |  Feature Depending on Ticket:
         Total Hours:  0             |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 This is a follow up task of #1371/#1372.  Now we have various types
 of xfr response stream (AXFR, incremental IXFR, single SOA as a
 special case of IXFR), it would be cleaner if we could generalize
 these cases and make _reply_xfrout_query() agnostic about the stream
 types.

 For example, we can introduce a new base class XfrStream (tentatively
 named) and define subclasses corresponding to the stream types.
 Each subclass knows special handling (whether or not to skip SOA, etc)
 specific to the types.  _reply_xfrout_query() itself simply takes
 an XfrStream object and creates messages from the stream unconditionally.

 I wanted to introduce this type of cleanup in #1371, but didn't choose
 it in that ticket to keep the diff minimize.

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Ticket URL: <http://bind10.isc.org/ticket/1398>
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