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          |  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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 Subtask of #2368.  Depend on #2377.

 See BIND 9's lib/dns/master.c:load_text().  Consider delegating to a
 separate method to keep each method manageable.

 "TTL guessing" (when TTL isn't explicitly specified for an RR) means
 something like:
 - if $TTL is defined use it
 - if there's a preceding RR, use its TTL
 - use SOA's minimum TTL.

 See BIND 9's implementation for details.

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Ticket URL: <http://bind10.isc.org/ticket/2429>
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BIND 10 Development