!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 
Sorry, I was thinking you had commented on the termination code for entries, not shards. I totally agree and will make the change.