Fix flushing and avoid upload caching

- Make update server always read from and write directly to S3 (via `useLocalFsCache` param on `FileLoader`).
- Make sure current flush method clears out WAVM and WAMR and SGX in-memory caches (as well as all locally cached files).
- Modify the quick-start test to add a re-upload of the same function (upload the `demo/echo` wasm to `demo/hello` and reinvoke, check that it's changed).