Add iterators for array requests

[fedekunze]

Keepers can use the store Iterator to retrieve all the items from the store by performing a callback function (eg an aggregator):

// IterateClients provides an iterator over all stored light client State
// objects. For each State object, cb will be called. If the cb returns true,
// the iterator will close and stop.
func (k Keeper) IterateClients(ctx sdk.Context, cb func(types.State) bool) {
	store := prefix.NewStore(ctx.KVStore(k.storeKey), types.KeyPrefixClient)
	iterator := sdk.KVStorePrefixIterator(store, nil)

	defer iterator.Close()
	for ; iterator.Valid(); iterator.Next() {
		var clientState types.State
		k.cdc.MustUnmarshalBinaryLengthPrefixed(iterator.Value(), &clientState)

		if cb(clientState) {
			break
		}
	}
}

Then you can use the Iterator function above to retrieve all the objects from the store by aggregating them:

// GetAllClients returns all stored light client State objects.
func (k Keeper) GetAllClients(ctx sdk.Context) (states []types.State) {
	k.IterateClients(ctx, func(state types.State) bool {
		states = append(states, state)
		return false
	})
	return states
}

Finally, we add the Querier method:

// QueryAllClients defines the sdk.Querier to query all the light client states.
func QueryAllClients(ctx sdk.Context, req abci.RequestQuery, k Keeper) ([]byte, error) {
	var params types.QueryAllClientsParams

	err := k.cdc.UnmarshalJSON(req.Data, &params)
	if err != nil {
		return nil, sdkerrors.Wrap(sdkerrors.ErrJSONUnmarshal, err.Error())
	}

	clients := k.GetAllClients(ctx)

	start, end := client.Paginate(len(clients), params.Page, params.Limit, 100)
	if start < 0 || end < 0 {
		clients = []types.State{}
	} else {
		clients = clients[start:end]
	}

	res, err := types.SubModuleCdc.MarshalJSON(clients)
	if err != nil {
		return nil, sdkerrors.Wrap(sdkerrors.ErrJSONMarshal, err.Error())
	}

	return res, nil
}

where QueryAllClientsParams contains only the pagination parameters:

type QueryAllClientsParams struct {
	Page  int `json:"page" yaml:"page"`
	Limit int `json:"limit" yaml:"limit"`
}

This is a pretty standard way on the SDK which would be helpful to support on the scaffold.