Fix the LoggerSilence to work as described:

[Edouard-chin]

• Following the Rails guide which state that a logger needs to include
  the ActiveSupport::LoggerSilence as well as
  ActiveSupport::LoggerThreadSafe modules isn't enough and won't
  work.

  Here is a test cases with 3 tests that all fails
  https://gist.github.com/Edouard-chin/4a72930c2b1eafbbd72a80c66f102010

  The problems are the following:

  1. The logger needs to call after_initialize in order to setup
     some instance variables.
  2. The silence doesn't actually work because the bare ruby Logger
     add method checks for the instance variable @logger. We need to
     override the add (like we used to in the ActiveSupport::Logger
     class).
  3. Calling debug? info? etc... doesn't work as the bare ruby
     methods will check for the instance variable. Again we need to
     override this methods (like we used to in the ActiveSupport::Logger
     class)

  The LoggerSilence won't work without LoggerThreadSafe, but the later
  is not public API, the user shouldn't have to include it so I
  modified to include it automatically.
  Same for the after_initialize method. I find unuintitive to have
  to call it directly. I modified to instance the variables when the
  module get included.

cc/ @rafaelfranca

[rails-bot]

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