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 (closed)

Signed-off-by: Kevin Morris kevr@0cost.org