chore: avoid double susbcription to the same cluster-id/shard

[Ivansete-status]

Background

By configuring a certain [cluster-id, shard], the node gets automatically subscribed (if relay mounted) to that cluster-id and shard.

If, after that, a manual subscription happens to the same [cluster-id, shard] combination, then we end up having to subscriptions. Then, in case of the node receiving a relay message, the msg-handlr-function is invoked twice.

We need to only have one subscription regardless the number of subscriptions happens to the same [cluster-id, shard] combination.