Add message stats to reporting (#2748)

Since we're now listening on the `OutputRoomEvent` stream, we are able
to store messages stats.

userapi/types/statistics.go

@@ -28,3 +28,10 @@ type DatabaseEngine struct {
 	Engine  string
 	Version string
 }
+
+type MessageStats struct {
+	Messages         int64
+	SentMessages     int64
+	MessagesE2EE     int64
+	SentMessagesE2EE int64
+}