* Test case covering changing shard id extractor with remember-entities
* This should do the trick
* Feedback addressed
* Docs and migration guide mention
* Correct logic to persist that entity has moved off off shard
Two new message pairs:
`GetShardRegionState`/`CurrentShardRegionState` allows for querying a region for it's current shards and the current `EntityIds` of it
`GetClusterShardingStats`/`ClusterShardingStats` allows for querying the entire cluster for a summary of
the number of entitites alive in each region and shard.