implement queue in data storage

## Background

In Gorse v0.4.x, neighbors, and models are not updated until the next round of full dataset loading. In practice, it would be better to update neighbors or models once a new user, item, or feedback is inserted. A queue is required to achieve this goal.

## Design

```go
type Database interface {

// Enqueue put the value to the back of a named queue.
Enqueue(name, value string) error

// Dequeue remove the front value of a named queue and return this value.
// If no value in the queue, return io.EOF.
Dequeue(name string) (value string, err error)
}
```