Ensure that file changes propogate immediately

Set the `Cache-control: no-cache` header to ensure that the content
store client does not cache the fetched items.

Always read the file from disk so that changes propogate immediately.
Move dummy server to `dummy_content_store` folder

This feels nicer as a self-contained app.

It can be run either using the `startup.sh` shell script, or as a rack
up directly using rackup and the `config.ru` file.