Losing the connection to Postgres stops the background jobs

[matrixbot]

This issue was originally created by @sandhose at matrix-org/matrix-authentication-service#2625.

When the connection to the database is lost, the background workers stop getting new jobs, even when the connection is back. There is no good indication when this happens, and the only way to fix it is to restart the service.

We should investigate to make the background jobs more reliable