Starts the underlying persistence engine and makes guice-persist ready for
use. For instance, with JPA, it creates an EntityManagerFactory and may
open connection pools. This method must be called by your code prior to
using any guice-persist or JPA artifacts. If already started,
calling this method does nothing, if already stopped, it also does
Stops the underlying persistence engine. For instance, with JPA, it
closes the EntityManagerFactory. If already stopped, calling this
method does nothing. If not yet started, it also does nothing.