Use SocketAppender and slf4j as logging api

- Deprecates com.findwise.hydra.Logger (simply use slf4j instead)
- Uses SocketAppender in the stages and SimpleSocketServer in the core
- Removes InternalLogger class. Use LoggerFactory.getLogger("internal") instead.
Various fixes and improvements to the new Logging API.

Also makes sure that ConsoleAppender can be turned on again if the situation requires it.