crypto: Add some TODOs for the key query handling.

matrix_sdk_crypto/src/machine.rs

@@ -679,6 +679,15 @@ impl OlmMachine {
         &self,
         response: &KeysQueryResponse,
     ) -> OlmResult<(Vec<ReadOnlyDevice>, Vec<UserIdentities>)> {
+        // TODO create a enum that tells us how the device/identity changed,
+        // e.g. new/deleted/display name change.
+        //
+        // TODO create a struct that will hold the device/identity and the
+        // change enum and return the struct.
+        //
+        // TODO once outbound group sessions hold on to the set of users that
+        // received the session, invalidate the session if a user device
+        // got added/deleted.
         let changed_devices = self
             .handle_devices_from_key_query(&response.device_keys)
             .await?;