!con #15496 Remember entries in cluster sharding

- 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.
- Shard buffers messages after an EntryStarted is received until state persisted
- Shard buffers messages (still) after a Passivate is received until state persisted
- Shard will retry persisting state until success
- Shard will restart entries automatically (after a backoff) if not passivated and remembering entries
- Added Entry path change to the migration docs