BIND 10 #2268: some cleanups for in-memory zone load

[BIND 10 Development]

#2268: some cleanups for in-memory zone load
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  medium        |                    Milestone:  Next-
           Component:  data source   |  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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 See also #2267.  I suggested the following there:

 - separate .cc for the load
 - stop using pimpl
 - introduce a helper class for loader

 In addition to these, I propose the following:

 - reuse the same `RdataEncoder` throughout the entire load().  If we
   use a helper class as suggested above, that would be a member of it.
 - sort `RdataSet`s in the order of type code.  maybe it's a premature
   optimization, but I guess younger code types (A, NS, SOA, etc, in
   particular A) are looked up more often than minor ones.  Since we
   use naive linear search, it's generally better to place such types
   of `RdataSet`s before minor ones.

 Maybe a beyond of 'hardening', but I would have at least discussed
 them in review if I had been there, so I'm proposing this ticket for
 the hardening sprint as a kind of delayed review feedback.

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Ticket URL: <http://bind10.isc.org/ticket/2268>
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