!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

akka-contrib/src/test/java/akka/contrib/pattern/ClusterShardingTest.java

@@ -63,7 +63,7 @@ public class ClusterShardingTest {
     //#counter-extractor
 
     //#counter-start
-    ActorRef startedCounterRegion = ClusterSharding.get(system).start("Counter", Props.create(Counter.class),
+    ActorRef startedCounterRegion = ClusterSharding.get(system).start("Counter", Props.create(Counter.class), false,
               messageExtractor);
     //#counter-start
 
@@ -111,12 +111,12 @@ public class ClusterShardingTest {
     }
 
     int count = 0;
-    
-    // getSelf().path().parent().name() is the type name (utf-8 URL-encoded) 
+
+    // getSelf().path().parent().parent().name() is the type name (utf-8 URL-encoded)
     // getSelf().path().name() is the entry identifier (utf-8 URL-encoded)
     @Override
     public String persistenceId() {
-      return getSelf().path().parent().name() + "-" + getSelf().path().name();
+      return getSelf().path().parent().parent().name() + "-" + getSelf().path().name();
     }
 
     @Override