Fix for LOGBACK-266 - FixedWindowRollingPolicy MAX_WINDOW_SIZE too small

Fix for http://jira.qos.ch/browse/LOGBACK-266

I've made the limit overridable by subclasses, and increased the limit from 12 to 20.

However, maybe the limit should be much higher like 100, since some filesystems can do very fast renames (unlike NTFS)..  But I didn't want to be too bold so just increased to 20.