func (c *Cluster) Transaction(f func(*ClusterTx) error) error {
	c.mu.RLock()
	defer c.mu.RUnlock()
	return c.transaction(f)
}
func (c *Cluster) Transaction(f func(*ClusterTx) error) error {
	c.mu.RLock()
	defer c.mu.RUnlock()
	return c.transaction(f)
}
// Transaction creates a new ClusterTx object and transactionally executes the
// cluster database interactions invoked by the given function. If the function
// returns no error, all database changes are committed to the cluster database
// database, otherwise they are rolled back.
//
// If EnterExclusive has been called before, calling Transaction will block
// until ExitExclusive has been called as well to release the lock.