message: Log slots in emit_batches to ease debugging
diff: @@ -103,7 +103,8 @@ public:
  template <class T>
  void emit_batches(local_actor* self, std::vector<T>& cache,
  bool force_underfull) {
  - CAF_LOG_TRACE(CAF_ARG(cache) << CAF_ARG(force_underfull));
  + CAF_LOG_TRACE(CAF_ARG(slots) << CAF_ARG(open_credit) << CAF_ARG(cache)
  + << CAF_ARG(force_underfull));
  if (pending())
  return;
  CAF_ASSERT(desired_batch_size > 0);
  