adding design for session monitoring API to transport plugins; skeleton implementation for UNIX plugin of that API; clean up of UNIX API code (removing of duplicated/dead code)

git-svn-id: https://gnunet.org/svn/gnunet@33756 140774ce-b5e7-0310-ab8b-a85725594a96
-fix doxygen

git-svn-id: https://gnunet.org/svn/gnunet@33757 140774ce-b5e7-0310-ab8b-a85725594a96