Filter RRs on specific event types.

[Velin92]

On some clients, like Element X we would like to still keep state events, but not show read receipts on them, however this would create an issue where RRs would magically disappear. Since we are working on a filter that hides the state event, but is capable of moving the RR on the closest event, would be possible to improve this API further (or create a similar one) were the filtered event does not hide but has its RR moved to the closest non filtered event so that we can have RRs only show for Messages if we want on a specific client.

[bnjbvr]

Thanks for opening an issue, I'll take a stab at it.