We don't necessarily have `instance_name` for old events (before we
support multiple event persisters). We treat those as if the
`instance_name` was "master".
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Co-authored-by: Eric Eastwood <eric.eastwood@beta.gouv.fr>
Use fully-qualified `PersistedEventPosition` (`instance_name` and `stream_ordering`) when returning `RoomsForUser` to facilitate proper comparisons and `RoomStreamToken` generation.
Spawning from https://github.com/element-hq/synapse/pull/17187 where we want to utilize this change
During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.