Decrease auto counter in database when user fails to sign-up

[CarlosACJ55]

Right now, the query from AddUser in mysql.go is great at not creating a new user whenever something is missing from the request. However, say for example there is only a single user with id 1, when another user later comes in and submits 3 mistakes and 1 correct sign up, they are assigned id 5 instead of id 2. Please add the failed id to the open_id table so that it can get taken on the next attempt, or otherwise fix this bug.