!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 believe so yes, if a handoff is received by a Shard, that Shard will start ignoring everything apart from terminated messages, until handoff is complete, at which point the ShardCoordinator will reallocate that shard elsewhere.