BIND 10 #1791: update InMemoryZoneFinder::load() to support sqlite3 backend

[BIND 10 Development]

#1791: update InMemoryZoneFinder::load() to support sqlite3 backend
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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:  xfr
         Total Hours:  0             |  for in-memory
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 This is a subtask of #1787.

 In this task we add another signature to InMemoryZoneFinder::load().
 it takes `ZoneIterator(Ptr)` instead of string& (specifying a file name)
 and load the RRsets from the iterator to in-memory.

 Note that the in-memory implementation requires some limitation about
 how the RRsets are coming (e.g., an RRset must not be divided;
 RRSIGs must come after the covered RRset, etc).  So the loader may
 need some smartness such as doing intermediate buffering.

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Ticket URL: <http://bind10.isc.org/ticket/1791>
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