Attempt to decrypt last x events in all E2EE rooms when session backup is enabled.

Right now, if you refresh after an initial sync and then enable session backup, the room list still won't be sorted correctly because the _missingSessions in `RoomEncryption` got cleared in the refresh. We'd better just read the last x (5, or until we find a message event?) events and retry decryption of those, which should trigger sessiom backup fetching.