* one Replicator per configured role
* log LMDB directory at startup
* clarify the imporantce of the LMDB directory
* use more than one key to support many entities
* CoordinatedShutdown that can run tasks for configured phases in order (DAG)
* coordinate handover/shutdown of singleton with cluster exiting/shutdown
* phase config obj with depends-on list
* integrate graceful leaving of sharding in coordinated shutdown
* add timeout and recover
* add some missing artery ports to tests
* leave via CoordinatedShutdown.run
* optionally exit-jvm in last phase
* run via jvm shutdown hook
* send ExitingConfirmed to leader before shutdown of Exiting
to not have to wait for failure detector to mark it as
unreachable before removing
* the unreachable signal is still kept as a safe guard if
message is lost or leader dies
* PhaseClusterExiting vs MemberExited in ClusterSingletonManager
* terminate ActorSystem when cluster shutdown (via Down)
* add more predefined and custom phases
* reference documentation
* migration guide
* problem when the leader order was sys2, sys1, sys3,
then sys3 could not perform it's duties and move Leving sys1 to
Exiting because it was observing sys1 as unreachable
* exclude Leaving with exitingConfirmed from convergence condidtion
* 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.
