Akka.Cluster.Sharding: need to log warnings when being run with state-store-mode=persistence and InMemoryJournal is being used

[Aaronontheweb]

Is your feature request related to a problem? Please describe.

One of the causes to #6973, indicated by the user's reported configuration on #7285 (reply in thread), is that they were running Akka.Cluster.Sharding with the system defaults - which are inherently unsafe for production use.

Describe the solution you'd like

We should do two things:

1. Make state-store-mode=ddata the default - we'd already planned on doing this as part of v1.6.
2. Log an unmissable warning that the settings you are running with are unsafe and that you can fix it by either using DData or by using a real persistence implementation.

Describe alternatives you've considered

I've considered making DData the default sooner but won't feel comfortable doing that until our next minor release - that's the right "cadence" to disseminate the change.