Make `observed_peer_subnets` Fork aware

[dknopik]

observed_peer_subnets is currently only a bitmap of size 128, and is updated on subscription/unsubscription of any fork subnet.

This might cause issues: If a peer is to subscribed to both ssv.v2.42 and /ssv/mainnet/boole/42, it will mark 42 as not subscribed when the peer unsubscribes from ssv.v2.42 at the end of the grace period, even though it is still subscribed to /ssv/mainnet/boole/42.