Use PDU not *Event in HeaderedEvent (#3073)

Requires https://github.com/matrix-org/gomatrixserverlib/pull/376

This has numerous upsides:
 - Less type casting to `*Event` is required.
- Making Dendrite work with `PDU` interfaces means we can swap out Event
impls more easily.
 - Tests which represent weird event shapes are easier to write.

Part of a series of refactors on GMSL.

appservice/consumers/roomserver.go

@@ -190,7 +190,7 @@ func (s *OutputRoomEventConsumer) sendEvents(
 
 	// If txnID is not defined, generate one from the events.
 	if txnID == "" {
-		txnID = fmt.Sprintf("%d_%d", events[0].Event.OriginServerTS(), len(transaction))
+		txnID = fmt.Sprintf("%d_%d", events[0].PDU.OriginServerTS(), len(transaction))
 	}
 
 	// Send the transaction to the appservice.