Improve default shard rebalancing algorithm, #26012 (#26101)

* Improve default shard rebalancing algorithm, #26012

* Use rebalance-threshold=1 because it will give the best distribution,
  and previous default could result in too large difference between nodes
* Off by one error, difference > threshold vs >=
* Added more unit tests
* Note that in some cases it may still not be optimal, stopping more
  shards than necessary, but a different strategy that looks at more
  than most and least is out of scope for this issue. In practise
  those cases shouldn't matter much.
* Also note that the rebalance interval is by default 10 seconds,
  so typically shards will start up before next rebalance tick.
  It's intentionally a slow process to not cause instabilities by
  moving too much at the same time.

akka-cluster-sharding/src/multi-jvm/scala/akka/cluster/sharding/ClusterShardingSpec.scala

@@ -158,7 +158,7 @@ abstract class ClusterShardingSpecConfig(
         number-of-entities = 1
       }
       least-shard-allocation-strategy {
-        rebalance-threshold = 2
+        rebalance-threshold = 1
         max-simultaneous-rebalance = 1
       }
       distributed-data.durable.lmdb {