BIND 10 #1750: add "at origin" option in ZoneFinder::find() and support it in in-memory

[BIND 10 Development]

#1750: add "at origin" option in ZoneFinder::find() and support it in in-memory
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  critical      |                    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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 In the actual query processing we often need to get RRs at zone's
 origin: SOA for negative resonses and NS for successful answer from
 the zone.  Since the in-memory data source internally maintain a
 shortcut to the rbt node corresponding to the origin, we could
 probably make it a bit more efficient.

 My proposal is to add a new option to ZoneFinder::find(), say,
 `FIND_AT_ORIGIN`.  If it's specified, it's a hint that the qname
 should be the zone origin, and under the caller's responsibility the
 implementation can exploit the hint to optimize the search; if the
 implementation doesn't support this option, it can simply ignore it.

 I guess it improves a performance bit, but this may be marginal.  So
 we should perform some benchmark to decide whether we really want it.

 THere's no dependency for this task.

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Ticket URL: <http://bind10.isc.org/ticket/1750>
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BIND 10 Development