!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 
To be honest, this was my biggest question in my head while developing this patch. In the end I took the stance that we have a message specify designed to say "I'm want to shut down". So my question was what do we do in the case it shuts down without sending that message... and the answer for me was restart it, due to the fact the actor didn't inform us of the intention to shutdown.

But I'm happy to change this to what ever you think is best, I wasn't 100% confident with that choice.