Issue #299 : Add transaction for deleteAccount()

Since the resetAccount() function has changed a fair bit, have a look at the before and after:

* before : https://github.com/chilts/fxa-auth-server/blob/master/db/mysql.js#L512-L537
* after : https://github.com/chilts/fxa-auth-server/blob/issue-299-add-transactions/db/mysql.js#L549-L567