Change @PluginVisitorStrategy to use a Class instead of a String.

  - I can't believe I forgot about this feature!
  - Also added some type-safety checking using wildcard bounds.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1597904 13f79535-47bb-0310-9956-ffa450edef68