Make historical events discoverable from backfill for servers without any scrollback history (MSC2716) (#10245)

* Make historical messages available to federated servers

Part of MSC2716: https://github.com/matrix-org/matrix-doc/pull/2716

Follow-up to https://github.com/matrix-org/synapse/pull/9247

* Debug message not available on federation

* Add base starting insertion point when no chunk ID is provided

* Fix messages from multiple senders in historical chunk

Follow-up to https://github.com/matrix-org/synapse/pull/9247

Part of MSC2716: https://github.com/matrix-org/matrix-doc/pull/2716

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Previously, Synapse would throw a 403,
`Cannot force another user to join.`,
because we were trying to use `?user_id` from a single virtual user
which did not match with messages from other users in the chunk.

* Remove debug lines

* Messing with selecting insertion event extremeties

* Move db schema change to new version

* Add more better comments

* Make a fake requester with just what we need

See https://github.com/matrix-org/synapse/pull/10276#discussion_r660999080

* Store insertion events in table

* Make base insertion event float off on its own

See https://github.com/matrix-org/synapse/pull/10250#issuecomment-875711889

Conflicts:
	synapse/rest/client/v1/room.py

* Validate that the app service can actually control the given user

See https://github.com/matrix-org/synapse/pull/10276#issuecomment-876316455

Conflicts:
	synapse/rest/client/v1/room.py

* Add some better comments on what we're trying to check for

* Continue debugging

* Share validation logic

* Add inserted historical messages to /backfill response

* Remove debug sql queries

* Some marker event implemntation trials

* Clean up PR

* Rename insertion_event_id to just event_id

* Add some better sql comments

* More accurate description

* Add changelog

* Make it clear what MSC the change is part of

* Add more detail on which insertion event came through

* Address review and improve sql queries

* Only use event_id as unique constraint

* Fix test case where insertion event is already in the normal DAG

* Remove debug changes

* Switch to chunk events so we can auth via power_levels

Previously, we were using `content.chunk_id` to connect one
chunk to another. But these events can be from any `sender`
and we can't tell who should be able to send historical events.
We know we only want the application service to do it but these
events have the sender of a real historical message, not the
application service user ID as the sender. Other federated homeservers
also have no indicator which senders are an application service on
the originating homeserver.

So we want to auth all of the MSC2716 events via power_levels
and have them be sent by the application service with proper
PL levels in the room.

* Switch to chunk events for federation

* Add unstable room version to support new historical PL

* Fix federated events being rejected for no state_groups

Add fix from https://github.com/matrix-org/synapse/pull/10439
until it merges.

* Only connect base insertion event to prev_event_ids

Per discussion with @erikjohnston,
https://matrix.to/#/!UytJQHLQYfvYWsGrGY:jki.re/$12bTUiObDFdHLAYtT7E-BvYRp3k_xv8w0dUQHibasJk?via=jki.re&via=matrix.org

* Make it possible to get the room_version with txn

* Allow but ignore historical events in unsupported room version

See https://github.com/matrix-org/synapse/pull/10245#discussion_r675592489

We can't reject historical events on unsupported room versions because homeservers without knowledge of MSC2716 or the new room version don't reject historical events either.

Since we can't rely on the auth check here to stop historical events on unsupported room versions, I've added some additional checks in the processing/persisting code (`synapse/storage/databases/main/events.py` ->  `_handle_insertion_event` and `_handle_chunk_event`). I've had to do some refactoring so there is method to fetch the room version by `txn`.

* Move to unique index syntax

See https://github.com/matrix-org/synapse/pull/10245#discussion_r675638509

* High-level document how the insertion->chunk lookup works

* Remove create_event fallback for room_versions

See https://github.com/matrix-org/synapse/pull/10245/files#r677641879

* Use updated method name

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