Fix retain cycle in MXKRoomDataSource

[Johennes]

This is the first part of a fix for a triple-leak involving MXKRoomDataSource, MXRoomEventTimeline and MXEventListener. The second part can be found in matrix-org/matrix-ios-sdk#1424.

The concrete issue here is that MXKRoomDataSource stores an event listener in a strong property but captures self in the listener's onEvent block.

I haven't found concrete steps to reproduce the leak (and the memory graph debugger is also not 100% reliable) but the cycle frequently shows after having entered, left and posted in rooms a few times.

Relates to: #5058

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[pixlwave]

Thanks @Johennes, these changes make sense to me 👍