Inconsistent file quota usage

We've currently disabled transactions on MySQL because they were causing concurrency problems, so a non-transaction solution would be preferrable.

It looks like currently, we update the user's quota by calling the User::quota_add($bytes) function, which adds a file's size to the usr.quotaused field. We could lessen the impact of this bug if we instead summed up the size of all the user's files. Preferrably in a single query, something like (although I'm not sure if this would be kosher in MySQL):

UPDATE usr u
SET quotaused = (
    SELECT sum(aff.size)
    FROM
        artefact_file_files aff
        INNER JOIN artefact a
            ON a.id = aff.artefact
    WHERE
        a.owner = u.id
)
WHERE
    u.id = ?

Of course you could still get race conditions, but they'd only last until the next pageload when we do "SELECT * FROM usr" (assuming that I am correctly remembering that we do that on each page and don't cache it in the session).