Add support for rolling logfile names in FileAppender (and Logger)

For long running applications, the `FileAppender` would only once create a filename for the logfile to append; this means, using a filename such as `myapp-%Y-%m-%d.log` would write to one file forever, until the app restarts.

This PR adds support for re-calculating the filename on every append, thus if a marker rolls over, it results in a new file being written to. Old behaviour can still be triggered by setting the new member `syncDate` to false (true is the new default).

/cc @haimich