Ensure we always have receipts for users we've seen
Ensure we always have receipts for users we've seen

This adds additional receipt storage to so that we can handle cases where the
receipts and events lists get out of sync. If we ever find a user who previously
had a receipt but momentarily no longer does, we recover their previous receipt
and go with that until we hear something new.

Part of https://github.com/vector-im/riot-web/issues/9745

matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk