BIND 10 #2358: DBGLVL_xxx should be in .cc, and in specific name space

[BIND 10 Development]

#2358: DBGLVL_xxx should be in .cc, and in specific name space
-------------------------------------+-------------------------------------
            Reporter:  jinmei        |                        Owner:
                Type:  defect        |                       Status:  new
            Priority:  medium        |                    Milestone:  Next-
           Component:  logging       |  Sprint-Proposed
           Sensitive:  0             |                     Keywords:
         Sub-Project:  Core          |              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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 `DBGLVL_xxx` constants defined in log_dbglevels.h break a couple of
 fundamental rules of C++.

 - It shouldn't be in unnamed namespace (`namespace {}`).  Having it in
   a shared header file easily leads to breaking the one definition
   rule.
 - It shouldn't be defined in the header file.  Since they are
   constants smart linkers may avoid generating duplicate copies, but
   as far as I know it's not guaranteed by the language.

 Solution is easy:

 - Declare them in a specific namespace, like isc::log
 - Only declare them in the header file; define them in a separate .cc:

 {{{#!cpp
 // in log_dbglevels.h
 // This is BAD
 //const int DBGLVL_START_SHUT = 0;
 extern const int DBGLVL_START_SHUT;
 }}}

 {{{#!cpp
 // in log_dbglevels.cc
 const int DBGLVL_START_SHUT = 20;
 }}}

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Ticket URL: <http://bind10.isc.org/ticket/2358>
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