While updating BAAS in https://github.com/realm/realm-core/pull/7042, the test "app: flx-sync basic tests" regularly hangs while waiting for a FLX subscription to be marked complete. This is reproducible locally.

A race in send_upload_message() causes subscriptions not being sent to the server (and as a side effect a notification on that subscription may never be fulfilled). This can happen when send_upload_message() is called as a result of a commit previous to the one for subscriptions. send_upload_message() then forwards the upload cursor past the version associated with the subscription and so the subscription is never uploaded to the server.