Add `event.internal_metadata.instance_name` (the worker instance that persisted the event) to go alongside the existing `event.internal_metadata.stream_ordering`.
`instance_name` is useful to properly compare and query for events with a token since you need to compare both the `stream_ordering` and `instance_name` against the vector clock/`instance_map` in the `RoomStreamToken`.
This is pre-requisite work and may be used in https://github.com/element-hq/synapse/pull/17293
Adding `event.internal_metadata.instance_name` was first mentioned in the initial Sliding Sync PR while pairing with @erikjohnston, see 09609cb0db (diff-5cd773fb307aa754bd3948871ba118b1ef0303f4d72d42a2d21e38242bf4e096R405-R410)
During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
There are a couple of things we need to be careful of here:
1. The current python code does no validation when loading from the DB,
so we need to be careful to ignore such errors (at least on jki.re there
are some old events with internal metadata fields of the wrong type).
2. We want to be memory efficient, as we often have many hundreds of
thousands of events in the cache at a time.
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Co-authored-by: Quentin Gliech <quenting@element.io>
