BIND 10 #352: Improve Xfrout request handling strategy

[BIND 10 Development]

#352: Improve Xfrout request handling strategy
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      Reporter:  zzchen_pku   |        Owner:  zhanglikun
          Type:  enhancement  |       Status:  reviewing 
      Priority:  major        |    Milestone:            
     Component:  xfrout       |   Resolution:            
      Keywords:               |    Sensitive:  0         
Estimatedhours:  0.0          |        Hours:  0         
      Billable:  1            |   Totalhours:  1.5       
      Internal:  0            |  
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Comment(by zhanglikun):

 there are three suggestion from jinmei:
 1. sleep(0.1) in the test case may not enough, the better way should to
 check whether self.__is_shut_down is set?
 2. remove the line server_thread.setDaemon(True) in the test case, since
 it's not used now.
 3. try to update the serve_forever()/shutdown() logic used by zonemgr and
 notify-out now(should be a seperate ticket, since ticket 335 has been
 closed). maybe it can share the same logic with this mixin.

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Ticket URL: <https://bind10.isc.org/ticket/352#comment:13>
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