BIND 10 #216: Xfrin: Implement the feature items in TODO file.

[BIND 10 Development]

#216: Xfrin: Implement the feature items in TODO file.
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      Reporter:  zhanglikun   |        Owner:  shentingting        
          Type:  enhancement  |       Status:  reviewing           
      Priority:  major        |    Milestone:  y2 6 month milestone
     Component:  xfrin        |   Resolution:                      
      Keywords:               |    Sensitive:  0                   
Estimatedhours:  0.0          |        Hours:  0                   
      Billable:  1            |   Totalhours:  5.0                 
      Internal:  0            |  
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Comment(by shentingting):

 some changes in xfrin code:
 1. Add config_handler function to make new config data to be applied.[TODO
 list #1]
 2. Delete unnecessary mutex on recorder, remove quato and repeat check to
 main thread.#2
 3. I checked that if xfrin fails, the garbage data is not in DB file. but,
 when all zone data are in the same zone file and multi zone xfrin process
 at the same time, there is data access conflicts.#7
 4. Deal with connection exception(the master closes connection etc.)
 during transmission in handle_read function#12
 5. Using socket pair communication mechanism to achieve quickly terminate
 on shutdown command, and still using a global flag. This is a double-check
 mechanism. I changed threading.Event() to a global flag to achieve the
 same function, this eliminates some confuse about thread.Event`s
 abuse.#13,#15,#16
 6. Amend test code.

 some changes after JinMei reviewed.
 1. delete self._socket.setblocking(1).
 2. change err = self._socket.connect_ex(address) to
 self._socket.connect(address), in order to use exception to deal with
 error.
 3. delete some comments and some useless lines.
 4. make _filter_hash() function more simpler.
 5. change two variables name. _conn_sockets rename
 _conn_sockets_to_threads
     _threads_zones rename  _zones_to_threads.

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Ticket URL: <https://bind10.isc.org/ticket/216#comment:23>
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