Expose logger mutex

If we expose the log dev mutex, we will be able to write new methods in a subclass of logger, and make sure they are thread safe, for instance:
```ruby
class FancyLogger < Logger
  def silence(temporary_level = Logger::ERROR)
    @logdev.synchronize do

    end
  end
end
```
related: https://github.com/rails/activerecord-session_store/issues/19

[fix https://bugs.ruby-lang.org/issues/9532]