BIND 10 #2565: Refactor RR param parsing code in MasterLoader

[BIND 10 Development]

#2565: Refactor RR param parsing code in MasterLoader
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            Reporter:  muks          |                        Owner:
                Type:  enhancement   |  jinmei
            Priority:  medium        |                       Status:
           Component:  libdns++      |  reviewing
            Keywords:                |                    Milestone:
           Sensitive:  0             |  Sprint-20130108
         Sub-Project:  DNS           |                   Resolution:
Estimated Difficulty:  0             |                 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):

 Replying to [comment:1 muks]:
 > `trac2565` branch is up for review.

 As we discussed in #2429, I suggest using boost::optional.  At least
 it doesn't make sense to use different approaches for `RRTTL` and for
 `RRClass` for essentially the same purpose.

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Ticket URL: <http://bind10.isc.org/ticket/2565#comment:2>
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