Fix retain cycles in MXEventListener & MXRoomSummary

[Johennes]

This is the second part of a fix for a triple-leak involving MXKRoomDataSource, MXRoomEventTimeline and MXEventListener. The first part can be found in element-hq/element-ios#5923.

There are two issues here:

1. When listening for events on timeline, MXRoomSummary strongly captures the latter in its own event handler block.
2. The sender property on MXEventListener is strong and, in all cases I was able to locate, the listener is stored in an array on the object that is passed in as sender, creating a retain cycle.

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: element-hq/element-ios#5058

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request contains a changelog file in ./changelog.d. See https://github.com/matrix-org/matrix-ios-sdk/blob/develop/CONTRIBUTING.md#changelog
• Pull request includes a sign off