BIND 10 #2380: revise b10-loadzone using datasrc.ZoneLoader

[BIND 10 Development]

#2380: revise b10-loadzone using datasrc.ZoneLoader
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  medium        |                    Milestone:  New
           Component:  loadzone      |  Tasks
           Sensitive:  0             |                     Keywords:
         Sub-Project:  DNS           |              Defect Severity:  N/A
Estimated Difficulty:  0             |  Feature Depending on Ticket:
         Total Hours:  0             |  loadzone-ng
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 Subtask of #2368, depend on #2379.

 This may not be super trivial, but I guess it's reasonably clear
 what should be done and how (if not, let's discuss it in comments).

 One note: this new loadzone isn't expected to create a new DB (see
 #1861).  it should fail if the DB doesn't exist.

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Ticket URL: <http://bind10.isc.org/ticket/2380>
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