Chunks should not load events from other rooms if they happen to be
requested for one eventId that already exists in a different room.
Motivation from a client that renders rich replies (although the
broken scenario can appear in other cases as well):
If somebody links an invalid eventId in a room, which however is valid
in a different room, this can mess up our timelines badly.
This can be reproduced by replying to an event in a room, then forward
the reply to a different room with a client that also forwards the
replied-to information (such as FluffyChat). Then click on the rich
reply to open the eventId. Previously, Android could find the event from
the other room and thus replace the correct timeline with the wrong one.
Compare e.g. https://matrix.to/#/!bfebJVBOZMnORmkVdO:matrix.org/$wUyRiMQEjaWOpJ-XpdBJzuXkh95N7bce2pVT4IMXW50?via=schildi.chat&via=matrix.org&via=envs.net
linking to an event that exists here
https://matrix.to/#/!SDwMepdfgrmExhyxYZ:schildi.chat/$MO2G4MZZ1zg0Ymc9gTfekIyw7QFkNn4OvYQKK1PAGlE
Change-Id: I4dcee94353d954fb6ed57c3970686a620b831c6f
