Support MSC4222 state_after (#4487)

* WIP support for state_after

* Fix sliding sync sdk / embedded tests

* Allow both state & state_after to be undefined

Since it must have allowed state to be undefined previously: the test
had it as such.

* Fix limited sync handling

* Need to use state_after being undefined

if state can be undefined anyway

* Make sliding sync sdk tests pass

* Remove deprecated interfaces & backwards-compat code

* Remove useless assignment

* Use updates unstable prefix

* Clarify docs

* Remove additional semi-backwards compatible overload

* Update unstable prefixes

* Iterate

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Iterate

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Fix test

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Iterate

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Iterate

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Add test for MSC4222 behaviour

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Improve coverage

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Iterate

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Fix tests

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Iterate

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Tidy

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Add comments to explain why things work as they are.

* Fix sync accumulator for state_after sync handling

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Add tests

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Revert "Fix room state being updated with old (now overwritten) state and emitting for those updates. (#4242)"

This reverts commit 957329b218.

* Fix Sync Accumulator toJSON putting start timeline state in state_after field

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Update tests

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Add test case

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

* Iterate

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

---------

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>
Co-authored-by: Hugh Nimmo-Smith <hughns@matrix.org>
Co-authored-by: Michael Telatynski <7t3chguy@gmail.com>
Co-authored-by: Timo <toger5@hotmail.de>

spec/unit/models/thread.spec.ts

@@ -801,7 +801,7 @@ async function createThread(client: MatrixClient, user: string, roomId: string):
 
     // Ensure the root is in the room timeline
     root.setThreadId(root.getId());
-    await room.addLiveEvents([root]);
+    await room.addLiveEvents([root], { addToState: false });
 
     // Create the thread and wait for it to be initialised
     const thread = room.createThread(root.getId()!, root, [], false);