!con #15496 Make entries of shards persistent

- Move all entry related logic out of the ShardRegion and into a
  new dedicated child `Shard` actor.
- Shard actor persists entry started and passivated messages.
- Non passivated entries get restarted on termination.
- Shard Coordinator restarts shards on other regions upon region failure or handoff
- Ensures shard rebalance restarts shards.

Refs #15496 
I'm not sure if it is important but the `persistenceId` of the coordinator is starting with `"/user/" + Settings.GuardianName` and this one is starting with `"/sharding"`. Perhaps we should change the coordinator persistenceId when moving things out of contrib. I created a ticket for that #15614.
Anyway, you can leave this as is.