rewrite state logic of gtp_path

* Move set of active context from state to data. Keeping them in there
  was not the best idea. Matching on set changes is better done by
  just keeping the count in there.
* Convert active context set to map. This allos us to stroe the monitoring
  key and demonitor them properly.
* Rework to Restart Counter change logic. now a restart counter change
  changes the state immediatly and processes all further GTP requests
  (incl. the one that triggered the RC change) in the new state.