Call once in the server function to set up the directory that
stream_path and stream_to_js will write to.
When running inside a shidashi template the directory is automatically
resolved from the template root; when running in plain Shiny a temporary
directory is created and registered with
addResourcePath so that the browser can fetch
stream/{token}_{id}.bin. An optional cleanup hook is registered
to remove this session's files when the session ends.
Usage
stream_init(session = shiny::getDefaultReactiveDomain())