[Filestore] Shards balancer crashes immediately after the filesystem resizes and increases shard count

[ruslan-gar]

1. TEvAggregateStatsCompleted was sent
2. The filesystem was resized, TIndexTabletActor::ExecuteTx_ConfigureShards was called,
   ShardBalancer was recreated with new number of shards.
3. TEvAggregateStatsCompleted was handled, UpdateShardBalancer with old number of shards was called,
   which caused Y_ABORT_UNLESS(stats.size() == Metas.size());

The solution is to ignore TEvAggregateStatsCompleted in TIndexTabletActor::StateZombie