Don't handle key verification requests which are immediately cancelled
Fixes https://github.com/vector-im/riot-web/issues/10083
Fixes https://github.com/vector-im/riot-web/issues/9197
Fixes https://github.com/vector-im/riot-web/issues/8629
The issue is partially fixed by https://github.com/matrix-org/matrix-react-sdk/pull/3123 in that users would no longer see "Incoming request", but would launch their client to a bunch of "key verification cancelled" dialogs. To work around this, we just don't handle key verification requests which we know are cancelled.
The changes are a bit awkward (flagging the event as cancelled instead of filtering it) because:
* We probably don't want to prevent events getting sent over the EventEmitter because applications may still rely on them.
* The cypto side only has visibility of 1 event at a time, so it needs to have some kind of flag to rely on.
An attempt has been made to generalize the new event flag for possible future cases.