Improve client-side read receipt support

[bnjbvr]

• sliding sync: Trigger a RoomList update when a room is updated b/o extensions #2996
• use sync ordering for read receipts (stash orphan read receipts, etc.)
  • done in Read receipts: use sync order when computing client-side read receipts #2978
• should_send_receipt seems buggy
  • investigated: it's actually correct, it was the computing of read receipts that wasn't
• consider providing a mark_as_read function that sends a read receipt for the last event in a room
  • not done: doesn't seem required
• The room read status integration test has been written and disabled in this pr, because it failed on the code coverage task. It needs further investigation to re-enable it as soon as possible, or at least only disable it for the code coverage task.
  • test: reenable the unread count test in code coverage #3044

Part of https://github.com/element-hq/element-internal/issues/531