message: Add `num_streams` function to stream multiplexer
diff: @@ -138,10 +138,15 @@ public:
  stream_multiplexer(local_actor* self, backend& service);
  /// Queries whether stream `x` is managed by this multiplexer.
  - bool has_stream(const stream_id& x) const {
  + inline bool has_stream(const stream_id& x) const {
  return streams_.count(x) > 0;
  }
  + /// Queries the number of open streams.
  + inline size_t num_streams() const {
  + return streams_.size();
  + }
  +
  protected:
  // Dispatches `x` on the subtype `T`.
  template <class T>
  