message: Fix gcc 4.8 error
  Using a mongoc_change_stream_t reference in a braced initializer yields "invalid initialization of
  non-const reference."
diff: @@ -33,7 +33,7 @@ class change_stream::impl {
  enum class state { k_pending, k_started, k_dead };
  explicit impl(mongoc_change_stream_t& change_stream)
  - : change_stream_{change_stream}, status_{state::k_pending}, exhausted_{true} {}
  + : change_stream_(change_stream), status_{state::k_pending}, exhausted_{true} {}
  // no copy or move
  impl(impl&) = delete;
  