add function to obtain number of subscribers to NC

git-svn-id: https://gnunet.org/svn/gnunet@34377 140774ce-b5e7-0310-ab8b-a85725594a96
define transport plugin monitor IPC format

git-svn-id: https://gnunet.org/svn/gnunet@34378 140774ce-b5e7-0310-ab8b-a85725594a96
implementing monitoring functionality in transport service

git-svn-id: https://gnunet.org/svn/gnunet@34379 140774ce-b5e7-0310-ab8b-a85725594a96