BIND 10 #2811: Make some DomainTree code updates

[BIND 10 Development]

#2811: Make some DomainTree code updates
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                   Reporter:  muks   |                 Owner:
                       Type:         |                Status:  new
  enhancement                        |             Milestone:  Next-Sprint-
                   Priority:         |  Proposed
  medium                             |              Keywords:
                  Component:  data   |             Sensitive:  0
  source                             |           Sub-Project:  DNS
               CVSS Scoring:         |  Estimated Difficulty:  0
            Defect Severity:  N/A    |           Total Hours:  0
Feature Depending on Ticket:         |
        Add Hours to Ticket:  0      |
                  Internal?:  0      |
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 This is a ticket to track some `DomainTree` code updates that I've made in
 a private branch.

 * The rebalance-after-insert code is reorganized to make it more
 straightforward to understand, and it's better commented.

 * There is a test that verifies that the tree is balanced, and doesn't go
 over the red-black theoretical limit. It constructs a random large
 million+ name zone and checks that the distance from every node to its
 subtree root is within limit. This is a check that was not implemented and
 we should have this as performance correctness proof.

 Though the deletion ticket #2750 is independent, the balance testcase will
 be useful for the deletion work.

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Ticket URL: <http://bind10.isc.org/ticket/2811>
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