[Bugs] #1211 UNSP: let the logger do the formatting

[SugarLabs Bugs]

#1211: let the logger do the formatting
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    Reporter:  sascha_silbe               |          Owner:  tomeu                      
        Type:  defect                     |         Status:  new                        
    Priority:  Unspecified by Maintainer  |      Milestone:  Unspecified by Release Team
   Component:  sugar-datastore            |        Version:  Unspecified                
    Severity:  Unspecified                |       Keywords:  r?                         
Distribution:  Unspecified                |   Status_field:  New                        
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 With the attached patch we delegate the string formatting to the logger
 instead of doing it ourselves.
 Advantages:
 a) It's more robust: Constructs like 'debug("foo %r bar" % x)' are quite
 common in our code and will break if x is a tuple. 'debug("foo %r bar",
 x)' will work regardless of the type of x.
 b) formatting only happens if we actually log something.

 I haven't measured the impact of b) because a) is much more important to
 me (happened a lot of times during the object_id => (tree_id, version_id)
 conversion). At least one project has asked for pylint to flag these
 occurences and gave performance as a reason for the change, though (so
 they mave have measured it).

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Ticket URL: <http://dev.sugarlabs.org/ticket/1211>
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