Delete unused and duplicate class.

  - This enum is actually a part of LifeCycle, and this particular enum is unused.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1615854 13f79535-47bb-0310-9956-ffa450edef68
Add javadoc intro.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1615855 13f79535-47bb-0310-9956-ffa450edef68
Add TODO comment.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1615856 13f79535-47bb-0310-9956-ffa450edef68
Add @doubt regarding difference between Appender and LogEventListener.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1615857 13f79535-47bb-0310-9956-ffa450edef68
Add javadoc intro.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1615858 13f79535-47bb-0310-9956-ffa450edef68