Option to flush and rollover active log files on shutdown

Even if only programatically, it would be useful to have a way of ensuring all async loggers are flushed and the active log files are rolled over to their final names when using RollingFileAppender.

The OnStartupTriggeringPolicy does the renaming of the active file on restart, but this doesn't cover the cases of instance termination where there is no subsequent restart. In our case this leads to the active files never being visible to daemon processes responsible for log shipping (which ignore the active filenames).