BIND 10 #1668: redo threaded stats tests with mocksessions

[BIND 10 Development]

#1668: redo threaded stats tests with mocksessions
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            Reporter:  jelte         |                        Owner:
                Type:  task          |                       Status:  new
            Priority:  major         |                    Milestone:  New
           Component:  statistics    |  Tasks
           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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 The current tests in stats/ contain a number of strangely interacting
 threads, and it can be very tricky to reliably add or modify tests. (for
 instance, for #640, we just spent an hour hunting down a race which was
 caused by a zero-timeout join(), but making that join() blocking causes
 almost all of the other tests to deadlock).

 It is highly probable that most, if not all, of these tests don't even
 need threads, and that mock sessions and modules would do just fine.

 So we need to refactor these tests and, if possible, remove the threads.
 If not possible, we should at least make the threading code such that it
 does not rely on timeouts to finish, but cleanly wind down each started
 thread.

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Ticket URL: <http://bind10.isc.org/ticket/1668>
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