shard index implementation

Add a Cache to ES backend struct

	ElasticSearchBackend struct {
		ElasticSearchConfig
		Client *elastic.Client
		Cache APICache // put a ShardsCache struct at ElasticSearchBackend initialization
	}

ES interface do not change

func (es *ElasticSearchBackend) CreateContact(contact *Contact) error { //unchanged signature
//…
	shard, err := es.Cache.GetUserShard(userId)
	if err != nil {
		//~
	}
	// make use of shard to PUT doc into ES
	
//…
}

Into …~cache~index

type ShardsCache struct {
    redis *RedisBackend
    cassa *CassandraBackend
}

// implements APICache
func (sc *ShardsCache) GetUserShard(userId UUID) (shard string, err error) {
    shard, err := redis.client.Get(userId.String()).String()
	if err != nil {
		//shard not in redis, get it from cassa
		shard, err := cassa.GetUserShard(userId) // TODO
		if err != nil {
		    //handle error
		}
		// return shard and put it into cache for nex time
		go func(user, shard string) {
		    redis.client.Set(user, shard, time.Minute*10)
		}(userId.String(), shard)
	}
    return shard, nil
}