Remove type casts to the ExtendedLogger interface from the public Logger
interface. This means that the ExtendedLogger must be pushed up to all
log4j-streams APIs. With this change, there are no casts to
ExtendedLogger, just like in the rest of Log4j. The only cast needed,
unless we change LogManager is
"(ExtendedLogger)LogManager.getLogger(...)". See the example in the
class comment for LoggerPrintWriter.