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.

roomserver/acls/acls.go

@@ -63,7 +63,7 @@ func NewServerACLs(db ServerACLDatabase) *ServerACLs {
 			continue
 		}
 		if state != nil {
-			acls.OnServerACLUpdate(state.Event)
+			acls.OnServerACLUpdate(state.PDU)
 		}
 	}
 	return acls
@@ -88,7 +88,7 @@ func compileACLRegex(orig string) (*regexp.Regexp, error) {
 	return regexp.Compile(escaped)
 }
 
-func (s *ServerACLs) OnServerACLUpdate(state *gomatrixserverlib.Event) {
+func (s *ServerACLs) OnServerACLUpdate(state gomatrixserverlib.PDU) {
 	acls := &serverACL{}
 	if err := json.Unmarshal(state.Content(), &acls.ServerACL); err != nil {
 		logrus.WithError(err).Errorf("Failed to unmarshal state content for server ACLs")