Previously a failure during e.g. MailboxType.create() would make the
user guardian fail, tearing down the whole system as a result. The cause
is a deep bug in handling ActorCell creation that we cannot really fix
anymore due to resulting changes in semantics, hence this fix only
targets top-level actors (where the observable difference is an
unambiguous improvement).
fixes#15947
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.
