provide a run-once mode that disables container reuse

Today we reuse containers for performance reasons. There are many advantages however to run-once that disables container reuse. Just as @tysonnorris is adding support for intra-container concurrency via an opt-in annotation, we can do the same to prevent container reuse for an action annotated accordingly to create a new container every time. This has the nice property of ensuring that every function starts from the same pristine environment.