Turn append_pdu args into struct PduBuilder

src/database.rs

@@ -125,7 +125,7 @@ impl Database {
         })
     }
 
-    pub async fn watch(&self, user_id: &UserId, device_id: &DeviceId) -> () {
+    pub async fn watch(&self, user_id: &UserId, device_id: &DeviceId) {
         let mut userid_prefix = user_id.to_string().as_bytes().to_vec();
         userid_prefix.push(0xff);
         let mut userdeviceid_prefix = userid_prefix.clone();