`build_finished` Postgres event will never be fired for the dependent builds.
For example, on our Hydra, the following query always returns increasing
numbers, even though all notifications have been delivered:
```
hydra=> select count(1) from builds where notificationpendingsince is not null;
count
-------
4583
(1 row)
```
Thus, we have to iterate over all dependent builds and mark their
`notificationpendingsince` as `null`, otherwise they will pile up until
the next restart of hydra-notify, when they will get delivered.
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