Enhanced db-prefill to spawn its own API and persona-faker instances

Modified db-prefill to use `fork()` to start up an instance of the API and persona-faker servers, then kill them on completion; this means that we can now run db-prefill without any prior setup (after `npm install`).

Note that if you have already run `npm install` recently (ie. persona-faker is already installed as a module), you will need to run `npm install` again so that persona-faker will send its callback message to notify db-prefill that it is ready to run.