* The old implementation would cap the pool size (both corePoolSize
and maximumPoolSize) to max-pool-size, which is very confusing
becuase maximumPoolSize is only used when the task queue is bounded.
* That resulted in configuring core-pool-size-min and core-pool-size-max
was not enough, because it could be capped by the default max-pool-size.
* The new behavior is simply that maximumPoolSize is adjusted to not be
less than corePoolSize, but otherwise the config properties match the
underlying ThreadPoolExecutor implementation.
* Added a convenience fixed-pool-size property.
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.
