GetRedisStore get the actual woking store.

size: maximum number of idle connections.

NewStoreWithDB - like NewStore but accepts `DB` parameter to select redis DB instead of using the default one ("0") Ref: https://godoc.org/github.com/boj/redistore#NewRediStoreWithDB.

NewStoreWithPool instantiates a RediStore with a *redis.Pool passed in.

SetKeyPrefix sets the key prefix in the redis database.