Documentation of PersistService does not match its implementation

[skuzzle]

The JavaDoc on its start() and stop() method states that nothing should happen if service is already started or stopped respectively. However, trying to call those methods twice results in an IllegalStateException being thrown by the implementation due to these checks:

public synchronized void start() {
    Preconditions.checkState(null == emFactory, "Persistence service was already initialized.");
    ...
}

public synchronized void stop() {
    Preconditions.checkState(emFactory.isOpen(), "Persistence service was already shut down.");
    ....