When figuring out which topological token to start a purge job at, we
need to do the following:
1. Figure out a timestamp before which events will be purged
2. Select the first stream ordering after that timestamp
3. Select info about the first event after that stream ordering
4. Build a topological token from that info
In some situations (e.g. quiet rooms with a short max_lifetime), there
might not be an event after the stream ordering at step 3, therefore we
abort the purge with the error `No event found`. To mitigate that, this
patch fetches the first event _before_ the stream ordering, instead of
after.
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