Enable logging for wai-servlet apps

@NickSeagull

Currently there is no an easy, tested way to log from wai-servlet apps
The appropriate features could be
- work with the minimum possible configuration
- integration with java logging, specially within servers
  - but avoiding the madness of java log frameworks
- use of existing haskell libraries
The options could be:
- Make any of the existent haskell libraries work, maybe through a wrapper that integrates java logging
  - @NickSeagull has tested Network.Wai.Middleware.RequestLogger in wai-extra, that uses in turn fast-logger and it works if you use logStdOut
- Create a new library, specific for eta to fulfill the features, in the spirit of clojure timbre
  - This could be a long time task
Other useful links
- https://stackoverflow.com/questions/6310961/how-do-i-do-logging-in-haskell
- https://github.com/wdanilo/haskell-logger
- http://hackage.haskell.org/package/hslogger
  - it can output Log4j's XMLLayout
- https://hackage.haskell.org/package/katip

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