Rename CompositeFilter.hasFilters() to isEmpty(), it is more similar to the Collection concept.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1596308 13f79535-47bb-0310-9956-ffa450edef68
Rename and adjust the various {start|stop}Filter() methods to {start|stop}() to play as life cycles. This means that AbstractAppender no longer needs its own start/stop and instead can rely its superclass to start and stop its filters.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1596318 13f79535-47bb-0310-9956-ffa450edef68
Move LOGGER static up to super class.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1596321 13f79535-47bb-0310-9956-ffa450edef68