Refactor plugin cache to its own class.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1585603 13f79535-47bb-0310-9956-ffa450edef68
Add some TODO notes to self.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1585606 13f79535-47bb-0310-9956-ffa450edef68
Small refactoring for creating plugin objects.

  - Working on extracting a builder for this.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1585614 13f79535-47bb-0310-9956-ffa450edef68