Broke in #5971
Basically the bug is that if get_current_state_deltas returns no new updates and we then take the max pos, its possible that we miss an update that happens in between the two calls. (e.g. get_current_state_deltas looks up to stream pos 5, then an event persists and so getting the max stream pos returns 6, meaning that next time we check for things with a stream pos bigger than 6)
* Fix background updates to handle redactions/rejections
In background updates based on current state delta stream we need to
handle that we may not have all the events (or at least that
`get_events` may raise an exception).
