Redaction gets stuck in pending events if unsynced target gets synced as redacted already

If a redaction is applied to an event on the server that we haven't synced yet (e.g. because of gap), and when it does come down sync we only receive the redacted version, and the redaction is not synced at all. The redaction remote echo is never received in this case and the redaction is never removed from the pending events. We need to look at the redacted_because value as well while looking for remote echos.