BIND 10 #2429: support $TTL and TTL guessing in dns::MasterLoader

[BIND 10 Development]

#2429: support $TTL and TTL guessing in dns::MasterLoader
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |  jinmei
            Priority:  medium        |                       Status:
           Component:  libdns++      |  accepted
            Keywords:                |                    Milestone:
           Sensitive:  0             |  Sprint-20121218
         Sub-Project:  DNS           |                   Resolution:
Estimated Difficulty:  3             |                 CVSS Scoring:
         Total Hours:  0             |              Defect Severity:  N/A
                                     |  Feature Depending on Ticket:
                                     |  loadzone-ng
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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Comment (by jinmei):

 trac2429 is ready for review.

 I've incorporated snap shot versions of some other branches as a base
 of this task, and the actual part to be reviewed is commits
 of and after 606ba69e360a2d841e9ec5eb7e5bd32d2c86f062.

 - The initial commit (606ba69) is an independent change that is needed
   in the main part of the branch.  This can be reviewed separately if
   that helps.
 - The main changes are from 9fecf9f to e2ff6e8.  It's generally a
   straightforward port of the BIND 9 implementation (although the BIND
   9 code is also complicated, so this may not really help for
   review).  The implementation is a bit complicated due to various
   corner cases, but I hope it's still bitable in terms of the code
   size and logic complexity.
 - There's one other, possibly controversial change: 33d80f8.  To
   support flexible ordering/omission of TTL/type/class, we need to
   test some candidate text for TTL that may turn out to be the RR
   class or type.  If we use the existing RRTTL constructor, this will
   involve expensive exception handling.  So I extended the RRTTL
   interface specifically for this type of case and use that interface
   in the loader.  The extension itself is generic, but may look ad hoc
   and a kind of premature optimization.  In that case we can discuss
   it separately.  Feature wise the implementation is complete without
   this commit.

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Ticket URL: <http://bind10.isc.org/ticket/2429#comment:4>
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