Room lists update is slow and blocks the UI

[manuroe]

Logs say:

2020-10-28 08:16:03.249 Riot[1332:479726] [RecentsDataSource] refreshRoomsSections: Done in 1107ms

This happens almost at every /sync because a new message refreshes the last message of a room summary, which refreshes the room list.

This is time to implement smart update of cells within a UITableViewCell: https://github.com/matrix-org/matrix-ios-kit/blob/v0.12.26/MatrixKit/Models/MXKDataSource.h#L168 instead of reloading all the table every time.

[manuroe]

#4384 will change the storage layer of room summaries. It will impact performance regarding this issue a lot.