message: Add getter for current filter
diff: @@ -59,13 +59,20 @@ public:
  // -- properties -------------------------------------------------------------
  - /// Sets the filter for `x` to `f` and inserts `x` into the appropriate lane.
  - /// @pre `x` is not registered on *any* lane
  + /// Sets the filter for `slot` to `filter`. Inserts a new element if `slot`
  + /// is a new path.
  void set_filter(stream_slot slot, filter_type filter) {
  CAF_LOG_TRACE(CAF_ARG(slot) << CAF_ARG(filter));
  state_map_[slot].filter = std::move(filter);
  }
  + /// Returns the filter for `slot`. Inserts a new element if `slot` is a new
  + /// path.
  + filter_type& filter(stream_slot slot) {
  + CAF_LOG_TRACE(CAF_ARG(slot));
  + return state_map_[slot].filter;
  + }
  +
  /// Returns the broadcast states for all paths.
  state_map_type& states() {
  return state_map_;
  