Implement support for removing (cleaning) all log handlers.

At the moment we use `_Logger.shutdown()` to remove all handlers. There are two issues with our approach:

 * the state of `logging` module attributes gets corrupted,
 * handler list per logger instance is not cleaned.

In order to fix this we need to implement support for `removeAllHandlers()` at logger instance level and call `_Logger.shutdown()` only once, when the application terminates.