Store: Before deleting shard verify that another node holds an active shard instance

Before removing shard physically from disk verify that another node in the cluster actually holds an active shard instance.
Same here, we need j<= numReplicas, which also makes me wonder if we want to validate in shardCanBeDeleted that the total number of shards in the routing table is what we expect it to be (we now only check for no shards at all)