BIND 10 #2309: define ZoneFinder::findAtOrigin()

[BIND 10 Development]

#2309: define ZoneFinder::findAtOrigin()
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |  jinmei
            Priority:  medium        |                       Status:
           Component:  data source   |  accepted
            Keywords:                |                    Milestone:
           Sensitive:  0             |  Sprint-20130205
         Sub-Project:  DNS           |                   Resolution:
Estimated Difficulty:  6             |                 CVSS Scoring:
         Total Hours:  0             |              Defect Severity:  N/A
                                     |  Feature Depending on Ticket:
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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Comment (by jinmei):

 trac2309 is ready for review.

 - In the first two commits, I introduced some unrelated cleanups.  See
   the commit log for details, but they are basically renaming files
   and separating a single large header file into two; no behavior
   change otherwise.
 - The third commit also has some amount of diff, but that's
   refactoring without changing the behavior.

 These should basically be trivial, so it may be easier if they get
 reviewed first separately from the rest of the branch.

 Changes from c99567a to e99a3dd are the main extension to
 `ZoneFinder`.  The implementation itself should be quite
 straightforward.  Regarding the design, I chose to add a knob for
 specifying min TTL only for the new at-origin method, considering the
 fact all of these are optional and it would be nicer if other
 data source implementations can simply use its default implementation
 and don't have to bother considering and implementing these.

 The last commit (bc66f6d) is the bug fix part.  This should be trivial.

 Proposed changelog entry:
 {{{
 556.?   [bug]           jinmei
         b10-auth now sets the TTL of SOA RR for negative responses to
         the minimum of the RR TTL and the minimum TTL of the SOA RDATA
         as specified in RFC2308; previously the RR TTL was always used.
         The ZoneFinder class was extended partly for implementing this
         and partly for allowing further optimization.
         (Trac #2309 and #2635, git TBD)
 }}}

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Ticket URL: <http://bind10.isc.org/ticket/2309#comment:11>
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