Outbound group sessions are invalidated upon reception of a duplicate state event

Start with update_state() in mautrix/client/state_store/abstract.py and its registration in mautrix/client/client.py, then trace handle_member_event() in mautrix/crypto/machine.py and its registration there. Compare the concurrent loading and comparison paths with the referenced commits. Done means duplicate membership events no longer invalidate outbound group sessions while genuine state changes still do.

Mautrix uses lazy membership loading, which causes synapse to oftentimes send state events to the client, that the client has already received. Upon reception of a state event, mautrix invalidates all outbound group sessions. This is the correct thing to do, but because synapse frequently sends duplicate state events, mautrix invalidates outbound group sessions all the time.

A fix for this has first been implemented in 72fb0f61f2a8cf8f062ddb8bacaa98ef4cbd0aa5. This fix relies on storing the most recent membership event as an attribute of the event and then later comparing this stored event to the received event and ignoring the received event if they are equal:
https://github.com/mautrix/python/blob/3e22e6192e2a9d208b9e7ea159b2169daa60a1ce/mautrix/client/state_store/abstract.py#L146-L148
https://github.com/mautrix/python/blob/3e22e6192e2a9d208b9e7ea159b2169daa60a1ce/mautrix/crypto/machine.py#L193-L200

The problem is, that loading the most recent membership event and comparing the stored membership event happens in different event handlers, which are executed concurrently.
The most recent membership event is loaded in the update_state() event handler, which is registered here:
https://github.com/mautrix/python/blob/3e22e6192e2a9d208b9e7ea159b2169daa60a1ce/mautrix/client/client.py#L31

Comparing the received membership event against the most recent one happens in handle_member_event(), which is registered here:
https://github.com/mautrix/python/blob/3e22e6192e2a9d208b9e7ea159b2169daa60a1ce/mautrix/crypto/machine.py#L109
While the update_state() handler is executed first, execution is interrupted once the most recent membership event is loaded via await self.get_member(), as this loads the event from the database, which takes some time. In the meantime, handle_member_event() becomes active and attempts to compare the received membership event against the most recent one, which hasn't yet been loaded from the database. As a result, the comparison fails and the outbound session is invalidated.

I implemented a hacky fix for this in https://github.com/jkhsjdhjs/mautrix-python/commit/e7c1921fb8f52549baf09ed2784fe46573964911, as I don't know a good way to fix this properly.