add key holder

[TcMits]

Hi @a8m

Inspired by cachops, i add key holders which created in onClose function. Each holder is equivalent with record(s). It make easier to evict keys, which are related to record(s).

Example:

expectQuery(evictCtx, t, drv, "SELECT name FROM users LIMIT 1 OFFSET 0", []interface{}{"a8m"}) // evicted
expectQuery(ctx, t, drv, "SELECT name FROM users", []interface{}{"a8m", "a9m"}) // evicted too
expectQuery(ctx, t, drv, "SELECT name FROM users LIMIT 1 OFFSET 1", []interface{}{"a9m"}) // still in cache

Its also related to issue #13. When update a record with UpdateOne, ent will fetch record after updating.

u := client.T.Get(ctx, id)
u, _ = u.Update().SetName("nottest").Save(entcache.Evict(ctx))

graph.go

	if !update.Empty() {
		var res sql.Result
		query, args := update.Query()
		if err := tx.Exec(ctx, query, args, &res); err != nil {
			return err
		}
		...
	}
	...
	rows := &sql.Rows{}
	query, args := selector.Query()
	if err := tx.Query(ctx, query, args, rows); err != nil {
		return err
	}
	return u.scan(rows)