fix: retry transactions who fail due to deadlocks

In my opinion, this kind of handling of transactions is pretty ugly.
The being said, we have issues with running into deadlocks on aur.al,
so this commit works against that immediate bug.

An ideal solution would be to deal with retrying transactions through
the `db.begin()` scope, so we wouldn't have to explicitly annotate
functions as "retry functions," which is what this commit does.

Closes #376

Signed-off-by: Kevin Morris <kevr@0cost.org>
2 jobs for db-deadlock-retry in 2 minutes and 9 seconds (queued for 2 seconds)
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