Exclusively join shards a peer is involved in after state change

[adlrocha]

When a state change is detected in the shard actor of a shard, we need to check if that state change involves our peer in some way:

• Either because we joined the shard and we are a new miner.
• Or because we staked to join the shard.

We are subscribed to every state change of the shard actor in the shards we are subscribed in, and we need to filter events conveniently.