Method setNumRows

bolt/exec/tests/CustomJoinTest.cpp:66–75

class CustomJoinBridge : public JoinBridge {
 public:
  void setNumRows(std::optional<int32_t> numRows) {
    std::vector<ContinuePromise> promises;
    {
      std::lock_guard<std::mutex> l(mutex_);
      BOLT_CHECK(!numRows_.has_value(), "setNumRows may be called only once");
      numRows_ = numRows;
      promises = std::move(promises_);
    }
    notify(std::move(promises));
  }

  std::optional<int32_t> numRowsOrFuture(ContinueFuture* future) {
    std::lock_guard<std::mutex> l(mutex_);