BIND 10 #2218: update InMemoryZoneFinder::findNSEC3 using memory-efficient version

[BIND 10 Development]

#2218: update InMemoryZoneFinder::findNSEC3 using memory-efficient version
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            Reporter:  jinmei        |                        Owner:
                Type:  defect        |                       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             |  scalable inmemory
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 (I've noticed this task should have been included for the feature
 but has been missing).

 After #2107 the NSEC3 name space will also be maintained in a
 `DomainTree`, not std::map (from std::string to rrset).  Likewie
 the zone's NSEC3 parameter won't be maintained in the form of
 NSEC3Hash object.  So we need to update the findNSEC3() method
 accordingly.  Doing something similar to getClosestNSEC(), etc.

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Ticket URL: <http://bind10.isc.org/ticket/2218>
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BIND 10 Development