BIND 10 #2823: complete removing threads from stats tests

[BIND 10 Development]

#2823: complete removing threads from stats tests
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                   Reporter:         |                 Owner:
  jinmei                             |                Status:  new
                       Type:         |             Milestone:  Next-Sprint-
  defect                             |  Proposed
                   Priority:         |              Keywords:
  medium                             |             Sensitive:  0
                  Component:         |           Sub-Project:  DNS
  statistics                         |  Estimated Difficulty:  0
               CVSS Scoring:         |           Total Hours:  0
            Defect Severity:  N/A    |
Feature Depending on Ticket:         |
        Add Hours to Ticket:  0      |
                  Internal?:  0      |
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 This is a followup of #2689.

 First read the analysis comments of #2689:
 http://bind10.isc.org/ticket/2689#comment:3

 For b10-stats.test.py, the remaining tests to be converted are:

 - test_init
 - test_commands
 - test_command_show
 - test_command_showchema
 - test_polling
 - test_polling2

 I believe we can eliminate the need for threads like other cases: use
 the `SimpleStats` mock class instead of the production `stats.Stats`,
 and directly pass/inspect messages over the `ModuleCCSession`.

 I guess the same approach can apply to b10-stats-httpd_test for things
 that rely on threaded `BaseModules`.  It (seemingly) uses threads also
 for http protocol handling.  If so, we need another mock or something
 to eliminate; at this moment I'm not sure if it's easy or difficult.
 If it makes the task too big, we can postpone that part further.

 Once we remove the dependency on `BaseModules`, we should be able to
 get rid of all of these things.  At that point I suggest rename
 `SimpleStats` (as it's not that simple any more).

 I'd also suggest renaming file names by removing "b10-" for
 consistency.

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Ticket URL: <http://bind10.isc.org/ticket/2823>
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BIND 10 Development