Feed events to widgets as they are decrypted (even if out of order)

[robintown]

The code that feeds events to widgets tries to enforce that only events from the end of the timeline will be passed through. This is to prevent old, irrelevant events from being passed to widgets as the timeline is back-filled. However, since encrypted events need to be decrypted asynchronously, it's not possible to feed them to a widget in a strictly linear order without introducing some kind of blocking or unreliable delivery. This code has been dropping events when they're decrypted out of order, which we consider to be an undesirable behavior.

The solution provided here is that, to reflect the asynchronous nature of decryption, encrypted events that arrive at the end of the timeline will be fed to a widget whenever they finish decrypting, even if this means feeding them out of order. For now we're not aware of any widgets that care about knowing the exact order of events in the timeline, but if such a need reveals itself later, we can explore adding ordering information to this part of the widget API.

Closes element-hq/element-call#2561
See also the previous discussion notes on matrix-org/matrix-react-sdk#12890

[robintown]

Draft as this is untested.