Change the way BURs are processed. Before, we spawned a background job
for each line of the BUR, then processed each job sequentially. Now, we
process the entire BUR sequentially in a single background job.
This means that:
* BURs are truly sequential now. Before certain things like removing
aliases weren't actually performed in a background job, so they were
performed out-of-order before everything else in the BUR.
* Before, if an alias or implication line failed, then subsequent alias
or implication lines would still be processed. This was because each
alias or implication line was queued as a separate job, so a failure
of one job didn't block another. Now, if any alias or implication
fails, the entire BUR will fail and stop processing after that line.
This may be good or bad, depending on whether we actually need the BUR
to be processed in order or not.
* Before, BURs were processed inside a database transaction (except for the
actual updating of posts). Now they're not. This is because we can't
afford to hold transactions open while processing long-running aliases
or implications. This means that if BUR fails in the middle when it is
initially approved, it will be left in a half-complete state. Before
it would be rolled back and left in a pending state with no changes
performed.
* Before, only one BUR at a time could be processed. If multiple BURs
were approved at the same time, then they would queue up and be
processed one at a time. Now, multiple BURs can be processed at the
same time. This may be undesirable when processing large BURs, or BURs
that must be approved in a specific order.
* Before, large tag category changes could time out. This was because
they weren't actually performed in a background job. Now they are, so
they shouldn't time out.
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