add api to bandwidth tracker to notify clients about excess bandwidth available

git-svn-id: https://gnunet.org/svn/gnunet@32263 140774ce-b5e7-0310-ab8b-a85725594a96
implementing #3296

git-svn-id: https://gnunet.org/svn/gnunet@32264 140774ce-b5e7-0310-ab8b-a85725594a96
-eliminate odd global variable, indentation, doxygen, stylistic fixes

git-svn-id: https://gnunet.org/svn/gnunet@32265 140774ce-b5e7-0310-ab8b-a85725594a96
implement #3295: only transmit background traffic if there is excess bandwidth

git-svn-id: https://gnunet.org/svn/gnunet@32266 140774ce-b5e7-0310-ab8b-a85725594a96