!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 think the safe choice would be to not start it again. I agree that users should use `Passivate` for graceful stop, but I'm afraid this behavior will be surprising if they use `stop`.
`stop` is not forbidden, it is just that some messages might go to deadletters, since the passivation buffer is not used.