* Expose reflex.utils.exec.is_prod_mode
Formalize runtime checking of the app's `--env` parameter.
* app_module_for_backend: wait for _compile in prod mode
Prod mode uses separate worker processes that fork from the main process.
If the app is not fully compiled when the fork occurs, any further changes to
the app (like mounting the _upload endpoint) will not be reflected in the
workers. This is not a performance hit because compile is skipped anyway
for backend processes and hot reload is not in the picture for prod mode.
* remove _is_dev_mode and replace it with calls to is_prod_mode()
_is_dev_mode was a private function in the compiler, but now that utils.exec
exposes is_prod_mode, we should use that throughout for consistency
