Optimise creation of DM (#16819)

* Optimise creation of dm

* Handle direct channels with the same user

* Cover GetMany with specs and add it on tha cache layer as well

* Fix specs by handling user dming themselves

* Apply PR suggestions

* Apply PR suggestions

* Use require.NoError instead of require.Nil on userstore test

* Improve readability of GetOrCreateDirectChannel

* Apply PR suggestions

* Update layers

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

store/sqlstore/user_store.go

@@ -325,6 +325,22 @@ func (us SqlUserStore) UpdateMfaActive(userId string, active bool) error {
 	return nil
 }
 
+// GetMany returns a list of users for the provided list of ids
+func (us SqlUserStore) GetMany(ids []string) ([]*model.User, error) {
+	query := us.usersQuery.Where(sq.Eq{"Id": ids})
+	queryString, args, err := query.ToSql()
+	if err != nil {
+		return nil, errors.Wrap(err, "users_get_many_tosql")
+	}
+
+	var users []*model.User
+	if _, err := us.GetReplica().Select(&users, queryString, args...); err != nil {
+		return nil, errors.Wrap(err, "users_get_many_select")
+	}
+
+	return users, nil
+}
+
 func (us SqlUserStore) Get(id string) (*model.User, error) {
 	query := us.usersQuery.Where("Id = ?", id)
 	queryString, args, err := query.ToSql()