State: add FetchSync option about arikawa HOT 6 CLOSED

func (s *State) Permissions(channelID, userID discord.Snowflake) (discord.Permissions, error) {
	if s.FetchSync {
		return s.permissionsSync(channelID, userID)
	}

	return s.permissionsAsync(channelID, userID)
}

func (s *State) permissionsSync(channelID, userID discord.Snowflake) (discord.Permissions, error) {
	ch, err := s.Channel(channelID)
	if err != nil {
		return 0, errors.Wrap(err, "Failed to get channel")
	}

	g, err := s.Guild(ch.GuildID)
	if err != nil {
		return 0, errors.Wrap(err, "failed to get guild")
	}

	m, err := s.Member(ch.GuildID, userID)
	if err != nil {
		return 0, errors.Wrap(err, "failed to get member")
	}

	return discord.CalcOverwrites(*g, *ch, *m), nil
}

func (s *State) permissionsAsync(
	channelID, userID discord.Snowflake) (discord.Permissions, error) {

	ch, err := s.Channel(channelID)
	if err != nil {
		return 0, errors.Wrap(err, "Failed to get channel")
	}

	guild, guildErr := make(chan *discord.Guild), make(chan error)
	member, memberErr := make(chan *discord.Member), make(chan error)

	go func() {
		g, err := s.Guild(ch.GuildID)
		if err != nil {
			err = errors.Wrap(err, "failed to get guild")
		}

		guild <- g
		guildErr <- err
	}()

	go func() {
		m, err := s.Member(ch.GuildID, userID)
		if err != nil {
			err = errors.Wrap(err, "failed to get member")
		}

		member <- m
		memberErr <- err
	}()

	g, err := <-guild, <-guildErr
	if err != nil {
		return 0, err
	}

	m, err := <-member, <-memberErr
	if err != nil {
		return 0, err
	}

	return discord.CalcOverwrites(*g, *ch, *m), nil
}

func (s *State) Permissions(channelID, userID discord.Snowflake) (discord.Permissions, error) {
	ch, err := s.Channel(channelID)
	if err != nil {
		return 0, errors.Wrap(err, "Failed to get channel")
	}

	var wg sync.WaitGroup

	g, guilderr := s.Store.Guild(ch.GuildID)
	if guilderr != nil {
		wg.Add(1)
		go func() {
			g, guilderr = s.Session.Guild(ch.GuildID)
			wg.Done()
		}()
	}

	m, membererr := s.Store.Member(ch.GuildID, userID)
	if membererr != nil {
		wg.Add(1)
		go func() {
			m, membererr = s.Store.Member(ch.GuildID, userID)
			wg.Done()
		}()
	}

	wg.Wait()

	if guilderr != nil {
		return 0, errors.Wrap(guilderr, "failed to get guild")
	}
	if membererr != nil {
		return 0, errors.Wrap(membererr, "failed to get member")
	}

	return discord.CalcOverwrites(*g, *ch, *m), nil
}