Introduce a new stream for the appservice consumer (#3277)

This introduces a new stream the syncAPI produces to once it processed a
`OutputRoomEvent` and the appservices consumes.
This is to work around a race condition where appservices receive an
event before the syncAPI has handled it, this can result in e.g. calls
to `/joined_members` returning a wrong membership list.

setup/jetstream/streams.go

@@ -20,6 +20,7 @@ var (
 	InputDeviceListUpdate   = "InputDeviceListUpdate"
 	InputSigningKeyUpdate   = "InputSigningKeyUpdate"
 	OutputRoomEvent         = "OutputRoomEvent"
+	OutputAppserviceEvent   = "OutputAppserviceEvent"
 	OutputSendToDeviceEvent = "OutputSendToDeviceEvent"
 	OutputKeyChangeEvent    = "OutputKeyChangeEvent"
 	OutputTypingEvent       = "OutputTypingEvent"
@@ -65,6 +66,11 @@ var streams = []*nats.StreamConfig{
 		Retention: nats.InterestPolicy,
 		Storage:   nats.FileStorage,
 	},
+	{
+		Name:      OutputAppserviceEvent,
+		Retention: nats.InterestPolicy,
+		Storage:   nats.FileStorage,
+	},
 	{
 		Name:      OutputSendToDeviceEvent,
 		Retention: nats.InterestPolicy,