Postgres VACUUMs hold back applying schema deltas

This issue has been migrated from [#10505](https://github.com/matrix-org/synapse/issues/10505).

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A user in the channel had the issue that a schema delta was blocked behind a vacuum.
Their logs didn't say anything other than Synapse was about to apply a schema delta.
In the end, they had to kill the autovacuum which got things going again.

Can we do something about this?

Ideas:

* cancel vacuums automatically (probably don't want to do this, from what I read)
* try to detect if a vacuum is running and warn in the logs so the administrator knows
* if a schema delta takes a long time, log a warning message that acknowledges this and maybe suggests checking if a vacuum is getting in the way