making the add method public to improve testability, but as a side effect,...

making the add method public to improve testability, but as a side effect, this makes this class mutable.

The concurrency commitment of the class needs to be defined eventually, or perhaps this class should be made immutable again.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@11571 71c3de6d-444a-0410-be80-ed276b4c234a