implement queue in data storage

[zhenghaoz]

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

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) 
}