Switch from a registry-based strategy to a meta-annotation-based strategy for associating visitors to annotations.

  - More easily extensible as there is no registry or special file to maintain.
  - Still maintains OSGi compatibility with correct ClassLoader usage.
  - Updated PluginVisitor(Builder|s) accordingly to use new algorithm.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1597790 13f79535-47bb-0310-9956-ffa450edef68