This relates to issue 2661 "public APIs must not reference private classes".

The classes in org.opends.messages need to be made public so that they can be referenced from other public APIs. This change adds the PublicAPI annotation to the classes defined in this package as follows:

   * the org.opends.messages package is VOLATILE

   * the org.opends.messages core classes (Message, MessageDescriptor,
     MessageBuilder, Category, and Severity) are all UNCOMMITTED

   * the class org.opends.messages.MessageDescriptorRegistry is PRIVATE
     (it's only used in unit tests)

   * generated message classes are PRIVATE

In addition minor cleanup was made to the classes where appropriate to make them suitable for public APIs:

   * used "final" where possible

   * reduced visibility of classes, methods, and members where appropriate

   * fixed some potential bugs spotted during this clean-up: class
     members not being initialized (the "final" keyword is useful for
     spotting this!)

   * added missing javadoc
   
   * I've removed the third-party and user-defined message generation
     since we could see know way in which they could be easily used
     without impacting core OpenDS source