Let ResilientOutputStreamBase.openNewOutputStream() be override from outside the package.

This method is currently package protected which means only subclasses in the same package can override this method. When other frameworks like Dropwizard want to provide their own implementation of a ResilientOutputStream they have to put their class in the ch.qos.logback.core.recovery package. Which causes a split package problem when using Java 9 modules: https://github.com/dropwizard/dropwizard/issues/2312

If this method is meant to be overridden from outside logback-core, a "protected" modifier should be added.