This fixes the following case when the backup is far behing the upstream DB,
and there are transactions being committed at the same time:

1. replicate partition 0
2. replicate partition 0
3. replicate partition 1
4. replicate partition 0
5. replicate partition 1
6. replicate partition 2
7. replicate partition 0
...
and so on in a quadratic way.

When the upstream activity was too high, the backup could even be stuck looping
on the first partitions.