BIND 10 #275: refactoring CC session class for easier tests

[BIND 10 Development]

#275: refactoring CC session class for easier tests
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 Reporter:  jinmei                      |       Owner:  jinmei                     
     Type:  enhancement                 |      Status:  new                        
 Priority:  major                       |   Milestone:  06. 4th Incremental Release
Component:  Inter-module communication  |    Keywords:                             
Sensitive:  0                           |  
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 I propose a refactoring based on r2393 (part of trac #221).

 The main idea is:
   - introduce a base abstract class of isc::cc::Session
   - other modules that use cc Session can develop tests more easily by
 using a derived mock Session class
   - it will be also useful if/when we want to add support for a "stand
 alone" mode in a particular process.  I can think of this feature for the
 auth server.
   - this is also a good opportunity to clean up non-asio mode of the
 Session class.  it will make the code much simpler
   - further, I'd make the config module independent from ASIO as part of
 this refacotring

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Ticket URL: <http://bind10.isc.org/ticket/275>
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